Biosensor and detection device

ABSTRACT

A TFT biosensor includes a gate electrode (silicon substrate), a reference electrode, and enzyme that is fixed to an insulating substrate spatially separated from the gate electrode and the reference electrode. A pH variation in the vicinity of an ion-sensitive insulating film is induced by a reaction between the enzyme and a sensing object material. The TFT biosensor can detect a concentration of the sensing object material with high sensitivity by detecting the pH variation as a threshold voltage shift of characteristics of a gate-source voltage to a source-drain current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.15/267,683, filed on Sep. 16, 2016, which claims priority under 35U.S.C. § 119(a) on Patent Application No. 2015-185236 filed in Japan onSep. 18, 2015, the entire contents of which are hereby incorporated byreference.

FIELD

The disclosure relates to a biosensor, and a detection device that usesthe biosensor.

BACKGROUND

Recently, the biosensor, which uses a biological material recognitionmechanism of biopolymer, has been used in medical and environmentalanalysis fields. The biosensor is obtained by combining the biologicalmaterial recognition mechanism of biopolymer, an interfacial potentialdetection mechanism at an interface (also referred to as a solid-liquidinterface) of a solution and an insulating film, and an electricalmeasurement device.

As the biological material recognition mechanism, substrate-specificityof enzyme, antibody-antigen reaction, a mutual operation betweendeoxyribonucleic acid (DNA) and DNA, a mutual operation betweenribonucleic acid (RNA) and RNA, coupling of lectin and physiologicalactive sugar chain, affinity of protein to specific biological material,and the like have been used.

As the interfacial potential detection mechanism, for example, an ionsensitive FET (FET sensor), in which a metal oxide semiconductor fieldeffect transistor (MOSFET) is set as basic structure, has been used. TheFET sensor measures an electric double-layer potential by detectingpotential variation of electric double-layer that is formed at thesolid-liquid interface as threshold voltage (Vth) shift of referenceelectrode potential-drain current characteristics (Vref-Idcharacteristics).

Examples of a main factor of causing the electric double-layer potentialto vary include a phenomenon such as variation in potential of hydrogen(pH) in the solution, physical and chemical adsorption to the insulatingfilm interface. For example, the relationship between the pH and theelectric double-layer potential is known by electrochemical Nernsttheory. For example, at 25° C., pH varies by 1 (this means that oneorder of magnitude of the hydrogen ion concentration in the solution ischanged). The electric double-layer potential varies by approximately 59mV due to the variation. This represents that 59 mV/pH is theoreticallimit of sensor sensitivity in the pH sensor based on the electricdouble-layer potential.

The level of pH is useful index for bio-sensing. The biosensor causesthe pH variation in the solution by decomposing the biological materialthrough enzyme reaction, and by generating hydrogen ions as aby-product. In addition, the biosensor measures a concentration of thebiological material by detecting the pH variation with the FET sensor.The biosensor has both molecular recognition and substrate decompositionfunction with enzyme, and pH measurement function with the FET sensor.Therefore, it is necessary for the molecular recognition and substratedecomposition function and the pH measurement function not to inhibiteach other. In addition, a variation in the concentration of hydrogenions that are generated through the enzyme reaction becomes lower than aconcentration of original biological material. Therefore, so as torealize bio-sensing in which the pH variation is set as index, it isnecessary to have a function capable of accurately detecting anextremely minute pH variation.

In the future, in clinical examination field, it is predicted that ademand for point of care testing (POCT), in which test is performed inthe vicinity of test subject in medical field, will increase. Thisclinical examination is performed to grasp the concentration of specificbiological material. In addition, in this clinical examination, it isconsidered that a demand for measurement of low-concentration material,which is not detected in an existing technology, will increase. To copewith this demand, the biosensor capable of performing high-sensitivitymeasurement is necessary.

Next, the descriptions will be given of technology (hereinafter,referred to as “related technology”) that relates to the disclosure.

With regard to the TFT biosensor, for example, there is reported caserelated to label-free detection of the DNA molecules and horseradishperoxidase molecules by using an amorphous silicon TFT (D. Goncalves,and three other persons, “Label-free electronic detection ofbiomolecules using a-Si:H field-effect devices”, “Journal ofNon-Crystalline Solids”, ELSEVIER, Jun. 15, 2006, volume 352, p.2007-2010). Furthermore, the TFT is an abbreviation of a thin filmtransistor. A linear Vth shift is obtained up to 0.4 μM in the DNAmolecules, and up to 0.1 μM in the horseradish peroxidase molecules.

In the TFT biosensor in which a carbon nanotube is used in an activelayer, there is disclosed an acetylcholine sensor in whichacetylcholinesterase is fixed to an upper portion of the active layer(Wei Xue, and other one person, “A thin-film transistor basedacetylcholine sensor using self-assembled carbon nanotubes and SiO₂nanoparticles”, “Sensors and Actuators B: Chemical”, ELSEVIER, Sep. 25,2008, volume 134, p. 981-987). As sensitivity, resolution, and responsetime, values of 378.2 μA/decade, 10 nM, and 15 seconds are obtained,respectively.

As known example in which the enzyme reaction is used, there is reportedthe penicillin sensor in which penicillin oxidase is fixed to anion-sensitive film of FET sensor (A. Poghossian, and other four persons,“An ISFET-based penicillin sensor with high sensitivity, low detectionlimit and long lifetime”, “Sensors and Actuators B: Chemical”, ELSEVIER,Jun. 1, 2001, volume 76, p. 519-526). The penicillin sensor has theconfiguration in which pH is allowed to vary by decomposing penicillinwith the enzyme, and by generating hydrogen ions as by-product, and thepH variation is detected by FET sensor. As detection sensitivity, 120±10mV/mM is obtained, and a continuous operation of one year or longer isconfirmed.

In addition, as biosensor including a field effect transistor, thefollowing case is reported. Specifically, a reaction field, to which adetection object material recognition molecule is fixed on one surfaceof a silicon substrate, and a field effect element which is formed onthe other surface of the silicon substrate as a detection unit, areprovided so as to attain an improvement in detection sensitivity(Japanese Patent Application Laid-Open No. 2013-148456).

In addition, there is disclosed an example of biosensor in which avertical transistor is used as transducer, and the enzyme and antibody,which have a molecule recognition function, are fixed to porous alumina,and which indicates a possibility of high-speed response operation(Japanese Patent Application Laid-Open No. 2010-151540).

SUMMARY

As described above, examples of the biosensor are disclosed. However,all of the above-described documents have the configuration in which agate voltage is applied by a reference electrode that is immersed in thesolution containing a measurement object material, and the concentrationof the measurement object material is measured from the Vth shift inVref-Id characteristics. In this sensor, it is difficult to obtainsensitivity that is higher than theoretical sensitivity that is based onNernst theory. As a result, it is difficult to apply the above-describedconfiguration to measurement of the biological material having anextremely low concentration.

The pH sensor using the basic structure of MOSFET is already inpractical use. When it is the try to applicate the measurement for thebiological materials, it is obvious that the higher sensitivity of pHsensor is needed. It is considered that the sensitivity is equal to orlower than 59 mV/pH based on the Nernst theory is sufficient in the caseof the pH measurement for the liquid solution.

When application is attempted to measurement of biological material, itis apparent that a high-sensitivity pH sensor is necessary. For example,a biological material, which becomes a measurement object of thebiosensor, exists in the solution having approximately pH 7 in theconcentration of approximately 10⁻⁷ to 10⁻⁹ mol/L. When the biologicalmaterial is decomposed with the enzyme, and a variation, which occurs asa result of the decomposition, in the concentration of hydrogen ions isdetected, it is necessary to detect a pH variation approximately in therange from 0.001 to 0.01. At this time, in the pH sensor of the relatedart, it is necessary to detect a minute voltage variation of 0.059 mV to0.59 mV. In biosensors of related technologies, it is difficult torealize high-reliability measurement when considering an effect such assensor drift, thermal fluctuation, a variation in a liquid temperature,and the like.

In addition, since the biopolymer is fixed onto the insulating film, thedisclosed technologies have the structure in which a materialrecognition unit with the biopolymer and a pH sensing portion with theFET sensor exist in the same portion. When the biopolymer film is madethick, the pH sensing portion does not come into contact with thesolution, and thus there is a concern that this situation leads to adecrease in pH sensitivity. Furthermore, a region in which the enzymecan be fixed is limited, and thus it is difficult to increase the amountof pH variation due to the enzyme reaction. That is, this leads to aproblem in which it is difficult to raise detection sensitivity for thebiological material.

In the structure of the disclosed technologies in which the enzyme isfixed onto the insulating film, basically, exchange of enzyme isdifficult. Typically, it is known that in the biopolymer such as enzyme,a function thereof deteriorate with the passage of time. Therefore, thefunction of the biopolymer is lost in a very short period of time incomparison to an inorganic structure body. Accordingly, in the structureof the disclosed technologies in which the exchange of enzyme isdifficult, when the enzyme is inactivated, even when the TFT sensor unitnormally operates, a function as the biosensor is lost. This leads to aproblem in which the lifetime of sensor is shortened, and a burden on auser increases.

In addition, in the configuration of the disclosed technologies, theenzyme molecule is disposed at a position that is most close to thereference electrode, and thus there is also a problem in that a decreasein activity may be caused due to application of gate voltage.

A biosensor according to an aspect of embodiments includes: asemiconductor active layer; a gate insulating film that is provided on afirst surface of the semiconductor active layer, and insulates thesemiconductor active layer and the gate electrode from each other; anion-sensitive insulating film that is provided on a second surface ofthe semiconductor active layer, and includes a region that comes intocontact with a solution; and enzyme that is fixed at a positionspatially separated from the region, and reacts with a material in thesolution to allow potential variation in the region to occur. Further,in the biosensor according to an aspect of embodiments, an electrostaticcapacity per unit area of the ion-sensitive insulating film is greaterthan an electrostatic capacity per unit area of the gate insulatingfilm.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a TFT biosensor of firstembodiment;

FIG. 2 is a graph illustrating Vg-Id characteristics of a TFT biosensorof Example 1;

FIG. 3 is a cross-sectional view illustrating a TFT biosensor of secondembodiment;

FIG. 4 is a schematic view of a part of the TFT biosensor in FIG. 3;

FIG. 5 is a cross-sectional view illustrating a TFT biosensor of Example3;

FIG. 6 is a cross-sectional view illustrating a TFT biosensor of thirdembodiment;

FIG. 7 is a cross-sectional view illustrating a TFT biosensor of fourthembodiment;

FIG. 8 is a cross-sectional view illustrating a TFT biosensor of Example6;

FIG. 9 is a cross-sectional view illustrating a TFT biosensor of fifthembodiment;

FIGS. 10A and 10B are cross-sectional views illustrating a TFT biosensorof sixth embodiment;

FIG. 11 is a schematic view illustrating the TFT biosensor of the sixthembodiment;

FIGS. 12A and 12B are cross-sectional views illustrating a TFT biosensorof seventh embodiment;

FIG. 13 is a cross-sectional view illustrating a TFT biosensor in whichan organic semiconductor is used as a semiconductor active layer;

FIG. 14 is a circuit diagram of a TFT biosensor device of eighthembodiment;

FIG. 15 is a view illustrating a configuration example of amicroprocessor of a TFT biosensor device;

FIG. 16 is an explanatory view illustrating a measurement principle inthe TFT biosensor device of the eighth embodiment;

FIG. 17 is a view illustrating a table in which correspondence between aconcentration of hydrogen ions and a gate electrode voltage is stored;and

FIG. 18 is an explanatory view of a measurement method in a TFTbiosensor device in Example 12.

DETAILED DESCRIPTION

Hereinafter, embodiments for carrying out the disclosure (hereinafter,referred to as “embodiments”) will be described with reference to theaccompanying drawings. Furthermore, in this specification and thedrawings, the same reference numeral will be given to substantially thesame constituent element. A shape in the drawings is illustrated foreasy comprehension of those skilled in the art, and does not entirelymatch actual dimensions and ratios.

In the following embodiments, the descriptions will be given ofbiosensor that is constituted by using TFT, and thus the biosensor willbe referred to as the TFT biosensor.

First Embodiment

FIG. 1 is the cross-sectional view illustrating a TFT biosensor 101 offirst embodiment.

The TFT biosensor 101 includes a semiconductor active layer 12 to whicha source electrode 13 s and a drain electrode 13 d are connected. Athermal oxide film 10 as a gate insulating film, and a silicon substrate11 as a gate electrode are provided on one surface (a first surface, alower surface in FIG. 1) of the semiconductor active layer 12. Inaddition, an ion-sensitive insulating film 14 and a protectiveinsulating film 15 are provided on the other surface (a second surface,an upper surface in FIG. 1) of the semiconductor active layer 12. Inaddition, the TFT biosensor 101 includes a reference electrode 17 at aposition that is spatially separated from the ion-sensitive insulatingfilm 14 and the protective insulating film 15.

Furthermore, an electrostatic capacity per unit area of theion-sensitive insulating film 14 is set to be greater than anelectrostatic capacity per unit area of the gate insulating film (thethermal oxide film 10). In addition, the TFT biosensor 101 includes asecond insulating substrate 18, in which enzyme 19 havingsubstrate-specificity is disposed on one surface, at a position that isspatially separated from the ion-sensitive insulating film 14 and theprotective insulating film 15. At this time, with regard to a structure,it is preferable that the silicon substrate 11 on which a TFT is formed,and the second insulating substrate 18 (enzyme 19) face each other asillustrated in FIG. 1, but may be two-dimensionally disposed such assame plane.

A space between the silicon substrate 11 and the second insulatingsubstrate 18 is filled with a solution that includes a sensing objectmaterial 16. The protective insulating film 15 covers a region otherthan a region, which overlaps with a channel region of the semiconductoractive layer 12, on an upper surface of the ion-sensitive insulatingfilm 14. The ion-sensitive insulating film 14 includes a region that isnot covered with the protective insulating film 15. At this region, theion-sensitive insulating film 14 comes into contact with the solutionthat includes the sensing object material 16.

In addition, it is preferable that a gap between the ion-sensitiveinsulating film 14 and the second insulating substrate 18 is as narrowas possible so as to make diffusion of hydrogen ions generated throughenzyme reaction fast, and so as to improve responsiveness of the TFTsensor. At an interface at which the ion-sensitive insulating film 14comes into contact with the solution that includes the sensing objectmaterial 16, the ion-sensitive insulating film 14 has properties ofallowing a potential in the interface to vary in response to apredetermined ion. The ion-sensitive insulating film 14 is also referredto as “ion-sensitive insulator”, or “pH-sensitive transducer”.

In addition, the TFT biosensor 101 further includes any one of a voltagedetection unit 20 that reads out potential difference between the sourceelectrode 13 s and a gate electrode (silicon substrate 11), and acurrent detection unit 21 that reads out a current that flows to thesource electrode 13 s or the drain electrode 13 d. Furthermore, in FIG.1, both the voltage detection unit 20 and the current detection unit 21are illustrated in the drawing.

Example 1

Next, Example 1, which further specifies the first embodiment, isdescribed using FIG. 1. First, the description is given of a method ofmanufacturing the TFT biosensor 101 of Example 1.

An apparatus of manufacturing the TFT biosensor 101 (hereinafter,referred to as “manufacturing apparatus”) forms the thermal oxide film10 on the silicon substrate 11 in a film thickness of 200 nm. A siliconoxide film, a silicon nitride film, and the like, which are formed byplasma chemical vapor deposition (CVD) method or sputtering method, maybe used instead of the thermal oxide film 10. Furthermore, the term of“manufacturing apparatus” is used as a generic name of individualapparatuses such as a film forming apparatus in sputtering or CVD, anapplicator of an organic material, and an annealing oven which arenecessary to manufacturing of biosensor.

In addition, the thermal oxide film 10 is formed on the siliconsubstrate 11, and the oxide semiconductor film composed ofindium-gallium-zinc-oxygen (hereinafter, omitted as In—Ga—Zn—O) isdeposited by sputtering method using metal mask. The thickness ofIn—Ga—Zn—O film is set at 50 nm. During the film formation, a sinteredbody target composed of In—Ga—Zn—O is used, the substrate is not heated,and a direct current (DC) sputtering method in mixed gas atmosphere ofargon gas and oxygen gas is employed. After the film formation, thesubstrate is annealed in the air at 400° C. for one hour. Thesemiconductor active layer 12 having an island shape is formed bypatterning the oxide semiconductor film.

Continuously, the source electrode 13 s and the drain electrode 13 d areformed by DC-sputtering of molybdenum (Mo) using the metal mask. Thefilm thickness of the source electrode 13 s and the drain electrode 13 dis set at 50 nm. In addition, the ion-sensitive insulating film 14, thatis, tantalum (Ta) oxide film having film thickness of 200 nm, issputtered and patterned by using metal mask. In the film formation, asintered body target composed of Ta—O is used, the substrate is notheated, and radio frequency (RF) sputtering method in mixed gasatmosphere of argon gas and oxygen gas is employed.

Then, the manufacturing apparatus performs annealing in the air at 300°C. for one hour. The specific dielectric constant of the thermal oxidefilm 10 is approximately 4, and the specific dielectric constant of thetantalum oxide (ion-sensitive insulating film 14) that is formed as afilm through sputtering is approximately 20. The film thickness of thethermal oxide film 10 and ion-sensitive insulating film 14 is set at 200nm, respectively. Accordingly, a difference in a value of the specificdielectric constant reflects on the electrostatic capacity per unitarea, and thus the electrostatic capacity per unit area of theion-sensitive insulating film 14 composed of the tantalum oxide isapproximately five times larger than the electrostatic capacity per unitarea of the gate insulating film constituted by the thermal oxide film10.

Continuously, the manufacturing apparatus exposes the surface of theion-sensitive insulating film 14 located directly over a channel regionof the semiconductor active layer 12, and covers the remaining part ofthe surface of the ion-sensitive insulating film 14 with the protectiveinsulating film 15. It is preferable that a silicone resin is used asthe protective insulating film 15, but a photoresist, an epoxy resin,and the like may be used as long as appropriate water resistance andinsulating properties are obtained.

The TFT having the structure described above is immersed in phosphatebuffered saline that includes the sensing object material 16. At thistime, the exposed region of the ion-sensitive insulating film 14 comesinto contact with the phosphate buffered saline. Furthermore, thephosphate buffered saline is an example of the solution. In addition, anAg/AgCl electrode that is filled with a saturated KCl solution is usedas the reference electrode 17, and is immersed in the phosphate bufferedsaline that includes the sensing object material 16.

For example, a main component of the enzyme 19 is glucose oxidase.Specifically, the enzyme 19 is a mixed material of 10% glucose oxidase,10% bovine serum albumin, and 8% glutaraldehyde. The manufacturingapparatus adds the enzyme 19 dropwise to one surface of the secondinsulating substrate 18, and dries the enzyme 19 at room temperature fortwo hours. Through the drying, the enzyme 19 is fixed to the secondinsulating substrate 18.

The enzyme 19 after the fixing is immersed in a phosphate buffersolution that is adjusted to pH 6.5 and 0.1 mol/L, and is kept at 4° C.The manufacturing apparatus immerses the second insulating substrate 18,to which the enzyme 19 is fixed, in the phosphate buffered saline thatincludes the sensing object material 16. Here, the second insulatingsubstrate 18 faces the silicon substrate 11 on which the TFT is formed.At this time, the second insulating substrate 18 and the siliconsubstrate 11 may be bonded to each other with a spacer interposedtherebetween so as to control a distance between two substrates. Thephosphate buffered saline is adjusted to pH 6.8 and a liquid temperatureof 37° C. as optimal environment of the enzyme 19.

In the TFT biosensor 101 that is configured as described above, first,the present inventors applied a constant potential of 0.5 V to the drainelectrode 13 d of the TFT biosensor 101, set the source electrode 13 sand the reference electrode 17 to a ground potential (0 V), and changeda gate voltage Vg to vary in a range of 0 V to +7 V so as to measureVg-Id characteristics (characteristics of a drain current Id to the gatevoltage Vg).

FIG. 2 is a graph illustrating the Vg-Id characteristics of the TFTbiosensor 101 of Example 1. A graph on an upper side of FIG. 2represents a measurement result of the Vg-Id characteristics in the air,and a graph on a lower side represents a measurement result of the Vg-Idcharacteristics in the phosphate buffered saline. It can be understoodthat the Vg-Id characteristics shift to a positive side due to immersionin the liquid. Next, the present inventors added a glucose aqueoussolution, which was adjusted in order for a final concentration to be apredetermined value, into the phosphate buffered saline. Furthermore,the TFT biosensor 101, the reference electrode 17, and the enzyme 19fixed on the second insulating substrate 18 are immersed in thephosphate buffered saline. At this time, the added glucose is dissolvedin the same phosphate buffered saline in order to keep pH of thephosphate buffered saline.

Here, the following reaction progresses between the glucose that isadded and the glucose oxidase (enzyme 19).β-D-glucose+O₂→D-glucono-δ-lactone+H₂O₂ (catalyst: glucose oxidase)

At this time, D-glucono-δ-lactone is converted into a gluconic acidthrough hydrolysis, and pKa (acid dissociation constant) of the gluconicacid is approximately 3.8, and thus the pH variation of solution iscaused. The pH variation increases in proportion to the concentration ofglucose in the solution. Accordingly, the TFT biosensor 101 is possibleto measure the concentration of glucose based on Vg-Id characteristicshift, which is caused by the pH variation.

In this example, a desired value is detected from Vth shift of the Vg-Idcharacteristics (the characteristics between the gate electrode voltageand the drain current). This is different from the related technologiessuch as the detection of interfacial potential or of ion concentrationfrom Vth shift according to the Vref-Id characteristics.

The TFT biosensor 101 of Example 1 includes a detection unit thatdetects potential difference (corresponding to an electric double-layerpotential that occurs at an interface), which occurs between theion-sensitive insulating film 14 and the sensing object material 16,after amplifying the potential difference with a value of a ratioobtained by dividing the electrostatic capacity per unit area of theion-sensitive insulating film 14 by the electrostatic capacity per unitarea of the gate insulating film (thermal oxide film 10) when thesensing object material 16 is disposed on the ion-sensitive insulatingfilm 14. For example, the detection unit reads out the potentialdifference obtained by multiplying the potential difference, whichoccurs on the ion-sensitive insulating film 14, by the value of theratio of the electrostatic capacity. The maximum value of a variation inthe electric double-layer potential to a variation in the concentrationof hydrogen ions is 59 mV/pH. However, in Example 1, the value of theratio obtained by dividing the electrostatic capacity per unit area ofthe ion-sensitive insulating film 14 by the electrostatic capacity perunit area of the gate insulating film (thermal oxide film 10) is greaterthan 1, and thus it is possible to realize sensitivity that is higherthan 59 mV/pH. That is, the TFT biosensor 101 of Example 1 is thebiosensor having pH sensitivity that is higher than 59 mV/pH.

In addition, the TFT biosensor 101 of Example 1 is biosensor includingbiomolecule recognition mechanism (for example, the enzyme 19) atspatially separated position from the ion-sensitive insulating film 14.

The meaning of the biomolecule recognition mechanism provided at thespatially separated position is that it can realize the high sensitivitybio-sensor described above without losing the configuration. When thebiomolecule recognition mechanism is applied to the biopolymer fixed tothe ion-sensitive film, it can be calculated that the pH sensitivity isdetermined by the value of ratio obtained by dividing an electrostaticcapacity per unit area of the biopolymer that is fixed to theion-sensitive film by the electrostatic capacity per unit area of thegate insulating film. Generally, the electrostatic capacity of thebiopolymer is very smaller compared with the ion-sensitive insulatingfilm. Therefore, in the case of fixing the biopolymer on the surfacewhere the area between the detected liquid material and theion-sensitive insulating film 14, it is difficult to realize the highsensitivity bio-sensor based on the principle of this embodiment.

In addition, it has the advantage caused by the independency of the pHmeasurement unit and the biological material recognition portion,because it brings the depression of interference between the pHmeasurement unit and the biological material recognition portion withoutthe mutual function interference. In biological material recognition, itis possible to realize a large area and a large thickness of biopolymerfixing portion without inhibiting the pH measurement function, and it ispossible to allow the enzyme reaction and molecule recognition reactionto efficiently progress. Accordingly, in the TFT biosensor 101 ofExample 1, efficiency of the biomolecular recognition mechanism isrealized while providing the interfacial potential detection functionwith high sensitivity. As a result, the TFT biosensor 101 can be appliedto measurement of a low-concentration biological material.

In this example, the sensing is performed from the threshold voltageshift (Vth shift) of the characteristics of voltage which is applied tothe gate electrode other than the reference electrode 17, to the draincurrent differently from the related technologies. In the case of usingthis detection method, when the electrostatic capacity per unit area ofthe ion-sensitive insulating film 14 is set to be greater than theelectrostatic capacity per unit area of the gate insulating film(thermal oxide film 10), detection with sensitivity higher than Nernstlimit is theoretically possible.

The effect of this example does not deny the Nernst theory, and isresult of “amplification” caused by the electric double-layer potentialdifference, which occurs on the surface of the ion-sensitive insulatingfilm 14 in accordance with the Nernst theory, through a mutual operationbetween a bottom gate electric field and a top gate electric field. Theamplification effect is realized by setting the electrostatic capacityper unit area of the ion-sensitive insulating film 14 to be greater thanthe electrostatic capacity per unit area of the gate insulating film(thermal oxide film 10). This effect does not depend on amplificationwith an external circuit and is not influenced by various kinds offluctuation, and thus intrinsic high sensitivity of the TFT biosensor101 is realized. As a result, it is possible to solve the problem of therelated technologies.

In this example, the description has been given of the example in whichglucose oxidase is used as the enzyme 19, but there is no limitationthereto. Any enzyme reaction may be employed as long as a pH variationof solution is caused.

In addition, when activity of the enzyme 19 decreases, the initialactivity of the enzyme 19 can be obtained again by replacing the secondinsulating substrate 18 and the enzyme 19 with new one while keeping aportion other than the substrate (second insulating substrate 18) of theenzyme 19. As a result, the lifetime of the entirety of the ion sensoris lengthened, and thus it is possible to provide as ion sensor that canbe used with small burden for the user.

In addition, the function capable of replacing the second insulatingsubstrate 18 and the enzyme 19 with new one is the same as aconfiguration in which substitution with another enzyme is possible. Dueto this replacement function, it is possible to provide a biosensorcapable of measuring other items with a single configuration.

The description has been given of the method of measuring the ionconcentration in the sensing object material 16 from the thresholdvoltage shift of the Vg-Id characteristics. In addition to this, whenthe user reads the sensor current between the source and the drainmeasured by ammeter at the fixed gate-source voltage, the sensor candetect the changing in the ion concentration.

Here, InGaZnO is used as the semiconductor active layer 12, but there isno limitation thereto. For example, amorphous silicon, polysilicon, ZnO,InSnZnO, and the like can be used. In addition, it is preferable to usea wide bandgap semiconductor in which a free hole is less likely to beaccumulated.

The ion-sensitive insulating film 14 is not limited to the tantalum (Ta)oxide, but it is preferable to use a material having a high specificdielectric constant. For example, in addition to the tantalum oxide(TaO₂), the material may be a hafnium oxide (HfO₂), an aluminum oxide(Al₂O₃), a barium titanate (BaTiO₃), a strontium titanate (SrTiO₃), asilicon nitride (Si₃N₄) film, and the like, and an arbitrary stackedfilm thereof is also possible. In addition, the gate insulating film isnot limited to the silicon oxide, and may be a silicon nitride, analuminum oxide, and the like, and an arbitrary staked film thereof isalso possible.

As described above, the ion sensor of this embodiment can detect theelectric double-layer potential, which occurs on the surface of theion-sensitive insulating film 14, after amplification thereof.Accordingly, it is possible to detect a minute variation in theconcentration of hydrogen ions, that is, the pH variation. In addition,it is possible to increase the amount of enzyme per unit area of the pHsensing portion. Accordingly, it is possible to make the amount of pHvariation due to the enzyme reaction large, and thus it is possible toraise detection sensitivity for the biological material.

Second Embodiment

FIG. 3 is a cross-sectional view illustrating a TFT biosensor 201 ofsecond embodiment.

As is the case with the TFT biosensor 101 of the first embodiment, theTFT biosensor 201 of the second embodiment includes the semiconductoractive layer 12 to which the source electrode 13 s and the drainelectrode 13 d are connected. In addition, a thermal oxide film 10 as agate insulating film, and a silicon substrate 11 as a gate electrode areprovided on one surface (a first surface, a lower surface in FIG. 3) ofthe semiconductor active layer 12. In addition, an ion-sensitiveinsulating film 14 and a protective insulating film 15 are provided onthe other surface (a second surface, an upper surface in FIG. 3) of thesemiconductor active layer 12. In addition, the TFT biosensor 201includes a reference electrode 17 at the spatially separated positionfrom the ion-sensitive insulating film 14 and the protective insulatingfilm 15. Furthermore, the electrostatic capacity per unit area of theion-sensitive insulating film 14 is set to be greater than theelectrostatic capacity per unit area of the gate insulating film (thethermal oxide film 10). In addition, in the TFT biosensor 201, in aregion, which is not covered with the protective insulating film 15, ona surface (upper surface) of the ion-sensitive insulating film 14, theenzyme 19 is fixed to a portion other than a portion located directlyover a region in which a lower surface of the ion-sensitive insulatingfilm 14 comes into contact with the semiconductor active layer 12.

In addition, the TFT biosensor 201 further includes any one of thevoltage detection unit 20 that reads out potential difference betweenthe source electrode 13 s and the gate electrode (silicon substrate 11),and the current detection unit 21 that reads out a current that flows tothe source electrode 13 s or the drain electrode 13 d. Furthermore, inFIG. 3, both the voltage detection unit 20 and the current detectionunit 21 are illustrated in the drawing.

Example 2

Next, Example 2, which further specifies the second embodiment, isdescribed using FIGS. 3 and 4. FIG. 4 is a schematic view of a part ofthe TFT biosensor 201 in FIG. 3. Reference numerals A1 to A4 in FIGS. 4and 5 are reference numerals which indicate regions, and are notreference numerals which indicate a material (such as the semiconductoractive layer 12, the ion-sensitive insulating film 14, the enzyme 19,and the like).

In the TFT biosensor 201 of Example 2, the thermal oxide film 10 isformed on the silicon substrate 11 in the film thickness of 200 nm. Asilicon oxide film, a silicon nitride film, and the like, which areformed by plasma chemical vapor deposition (CVD) method or sputteringmethod, may be used instead of the thermal oxide film 10.

An oxide semiconductor film composed of In—Ga—Zn—O and having the filmthickness of 50 nm is deposited on the silicon substrate 11 covered withthe thermal oxide film 10 by sputtering method. The oxide semiconductorfilm is patterned by the photo-resist process, and is etched by oxalicacid to form the semiconductor active layer 12 having a predeterminedisland shape. After formation of the semiconductor active layer 12, thesubstrate is annealed in the air at 400° C. for one hour.

Continuously, the titanium film is deposited by DC sputtering method,and the pattern is formed by photoresist process. And then, it is etchedby using fluorine gas-based plasma (for example, SF₆ or CF₄) to form thesource electrode 13 s and the drain electrode 13 d. At this time,In—Ga—Zn—O is not etched with the fluorine gas-based plasma, and thus itis possible to obtain a desired electrode shape without forming anetch-stop layer. Furthermore, the film thickness of the source electrode13 s and the drain electrode 13 d is set at 50 nm, respectively.

Next, the ion-sensitive insulating film 14 composed of hafnium oxide(HfO₂) with film thickness of 200 nm, is deposited by sputtering methodby using metal mask. In the film sputtering process, the sintered Hf—Otarget is used, the substrate is not heated, and RF sputtering method ina mixed gas atmosphere of argon gas and oxygen gas is employed. Andthen, the substrate is annealed in the air at 300° C. for one hour.Furthermore, the upper surfaced of the semiconductor active layer 12comes into the ion-sensitive insulating film 14 at a region (a secondregion A2 in FIG. 4) in which the source electrode 13 s and the drainelectrode 13 d are not formed.

A specific dielectric constant of the thermal oxide film 10 isapproximately 4, and a specific dielectric constant of the hafnium oxide(ion-sensitive insulating film 14) that is formed as film throughsputtering is around 20. The film thickness of the thermal oxide film 10and ion-sensitive insulating film 14 is 200 nm, respectively.Accordingly, a difference in the specific dielectric constant reflectson the electrostatic capacity per unit area, and thus the electrostaticcapacity per unit area of the ion-sensitive insulating film 14 composedof the hafnium oxide is approximately five times larger than theelectrostatic capacity per unit area of the gate insulating filmconstituted by the thermal oxide film 10.

Then, the surface (a first region A1 in FIG. 4) of the ion-sensitiveinsulating film 14 located directly over the channel region of thesemiconductor active layer 12 is exposed, and the surface of theperipheral edge portion is covered with the protective insulating film15 except for the first region A1. A polyimide resin is used as theprotective insulating film 15. The first region A1 includes a region (athird region A3 in FIG. 4) located directly over the second region A2 inwhich the semiconductor active layer 12 and the ion-sensitive insulatingfilm 14 come into contact with each other, and is formed to be greaterthan the second region A2. Next, the manufacturing apparatus fixesglucose dehydrogenase 19 to a fourth region A4 excluding the thirdregion A3 in the first region A1 of the ion-sensitive insulating film14. Protein including the glucose dehydrogenase 19 has a property ofbeing adsorbed to an oxide such as glass in a non-specific manner.

Therefore, the manufacturing apparatus adds an enzyme aqueous solutiondropwise to a target portion (the fourth region A4 of the ion-sensitiveinsulating film 14) and dries the enzyme aqueous solution. According tothis, the enzyme 19 is easily fixed to the target portion. In addition,for strong fixing of the enzyme 19, it is preferable to insert aself-assembled monolayer (SAM) film as a linker. Examples of a method offorming the SAM film include spin coating, dip coating, and vacuumdeposition, but there is no limitation thereto. As a material of the SAMfilm, a material that modifies a surface through a tiol group, or asilane coupling agent can be used. However, there is no limitationthereto as long as appropriate coupling strength is obtained.

As described above, in the ion-sensitive insulating film 14, theperipheral edge portion other than the first region A1 is covered withthe protective insulating film 15. The second surface (upper surface inFIG. 4) of the semiconductor active layer 12 includes the second regionA2 that comes into contact with the ion-sensitive insulating film 14.The first region A1 includes the third region A3 that overlaps with thesecond region A2, and the fourth region A4 other than the third regionA3. The enzyme 19 is fixed to the fourth region A4. The ion-sensitiveinsulating film 14 comes into contact with the solution in the thirdregion A3.

The fourth region A4 is located on the source electrode and the drainelectrode of the TFT, and thus potential variation in the fourth regionA4 does not have an effect on sensitivity. Accordingly, it is possibleto effectively use the fourth region A4, which does not effect on thesensitivity, as the region of the enzyme 19.

Example 3

Next, Example 3 as a modification example of the second embodiment isdescribed using FIG. 5. FIG. 5 is a cross-sectional view illustrating aTFT biosensor 301 of Example 3.

In the TFT biosensor 301 of Example 3, the thermal oxide film 10 isformed on the silicon substrate 11 in a film thickness of 200 nm. Next,a metallic chromium film is formed by DC sputtering method using metalmask, and the film is patterned to form a first gate electrode 22.Continuously, a silicon oxide film is formed in the mixed atmosphere ofargon gas and oxygen gas by RF sputtering method using metal mask inorder to form a first gate insulating film 23. In the film formation,any one of a silicon oxide and a metallic silicon may be used as atarget, and a desired withstand pressure is obtained by appropriatelycontrolling an oxygen partial pressure.

In addition, the oxide semiconductor film composed of In—Ga—Zn—O andhaving the film thickness of 50 nm is sputtered on the first gateinsulating film 23 using metal mask in order to form the semiconductoractive layer 12. After formation of the semiconductor active layer 12,the annealing process is performed in the air at 400° C. for one hour.Continuously, the source electrode 13 s and the drain electrode 13 d areformed by DC-sputtering of aluminum using metal mask. Furthermore, thefilm thickness of the source electrode 13 s and the drain electrode 13 dis set at 100 nm, respectively.

In addition, the ion-sensitive insulating film 14 composed of tantalumoxide (TaO₂) and having the film thickness of 200 nm is sputtered byusing metal mask. In the film formation process, a sintered body targetcomposed of Ta—O is used, the substrate is not heated, and RF sputteringmethod in a mixed gas atmosphere of argon gas and oxygen gas isemployed. Then, the annealing process is performed in the air at 300° C.for one hour. A specific dielectric constant of the thermal oxide film10 is approximately 4, and a specific dielectric constant of thetantalum oxide (ion-sensitive insulating film 14) that is formed as afilm through sputtering is approximately 20. The film thickness of thethermal oxide film 10 and ion-sensitive insulating film 14 is 200 nm,respectively. Accordingly, a difference in the specific dielectricconstant reflects on the electrostatic capacity per unit area, and thusthe electrostatic capacity per unit area of the ion-sensitive insulatingfilm 14 composed of the tantalum oxide is approximately five timeslarger than the electrostatic capacity per unit area of the gateinsulating film constituted by the thermal oxide film 10.

Then, the surface (the first region A1) of the ion-sensitive insulatingfilm 14 located directly over the channel region of the semiconductoractive layer 12 is exposed, and the protective insulating film 15 coversit except for the first region A1. A silicone resin is used as theprotective insulating film 15, but an inorganic insulating material suchas alumina may be used as long as appropriate water resistance andinsulating properties are secured.

Continuously, the manufacturing apparatus adds an enzyme aqueoussolution dropwise onto the first region A1 of the ion-sensitiveinsulating film 14 by using a dispenser, and dries the enzyme aqueoussolution at room temperature. According to this, the enzyme 19 is fixedonto the first region A1 of the ion-sensitive insulating film 14. Forexample, the enzyme 19 is provided on the first region A1 with a regularinterval or in a random manner. At this time, it is important that theion-sensitive insulating film 14 is appropriately exposed, and contactbetween the ion-sensitive insulating film 14 and the sensing objectmaterial 16 is secured. As a result, it is possible to provide a TFTbiosensor in which both the biological material recognition mechanismand the pH sensing mechanism are provided on the ion-sensitiveinsulating film 14 without the mutual functional interference.

As is the case with the TFT biosensor 101 of Example 1, in the TFTbiosensor 201 of Example 2 and the TFT biosensor 301 of Example 3, theVg-Id characteristics as illustrated in FIG. 2 is also obtained.Accordingly, it is possible to perform sensing from the thresholdvoltage shift of Vg-Id characteristics. In addition, even in Examples 2and 3, the TFT biosensors 201 and 301 are provided with a detection unitthat detects potential difference, which occurs between theion-sensitive insulating film 14 and the sensing object material 16,after amplifying the potential difference by multiplying the potentialdifference by the value of the ratio obtained by dividing theelectrostatic capacity per unit area of the ion-sensitive insulatingfilm 14 by the electrostatic capacity per unit area of the gateinsulating film (thermal oxide film 10). According to this, it ispossible to realize the biosensor having high pH sensitivity.

Third Embodiment

FIG. 6 is a cross-sectional view illustrating a TFT biosensor 401 ofthird embodiment.

The TFT biosensor 401 of the third embodiment includes the semiconductoractive layer 12 to which the source electrode 13 s and the drainelectrode 13 d are connected. The thermal oxide film 10 as the firstgate insulating film and the silicon substrate 11 as the first gateelectrode are provided on one surface (a first surface, a lower surfacein FIG. 6) of the semiconductor active layer 12. A second gateinsulating film 24 and a second gate electrode 25 are provided on theother surface (a second surface, an upper surface in FIG. 6) of thesemiconductor active layer 12.

An electrostatic capacity per unit area of the second gate insulatingfilm 24 is set to be greater than the electrostatic capacity per unitarea of the first gate insulating film (thermal oxide film 10). Thesecond gate electrode 25 includes a region, which overlaps with thesemiconductor active layer 12, on an upper surface of the second gateinsulating film 24, and is disposed to two-dimensionally (to a rightdirection in FIG. 6) extend from the region. That is, the second gateelectrode 25 is provided on the second gate insulating film 24, andextends from the region, which overlaps the semiconductor active layer12, to a position that is two-dimensionally separated from the region.

In addition, in the second gate electrode 25, the enzyme 19 is fixed toan upper surface on an extension end side, and a region other than theregion, to which the enzyme 19 is fixed, is covered with the protectiveinsulating film 15. The enzyme 19 reacts with a material in the solutionand modulates a voltage that is applied to the second gate electrode 25.

In addition, the TFT biosensor 401 includes the reference electrode 17at the spatially separated position from the enzyme 19 that is fixedonto the second gate electrode 25, and the protective insulating film15. A sensing object material 16 that is included in the solution isdisposed on the second gate electrode 25 and the enzyme 19, and avoltage form the reference electrode 17 is applied to the second gateelectrode 25. At this time, an effective gate voltage, which is appliedto the TFT biosensor 401, becomes a value obtained by adding a redoxpotential of enzyme reaction, which progresses on the second gateinsulating film 24, to the voltage of the reference electrode 17, and atop channel, which is caused by the effective gate voltage, is inducedto the semiconductor active layer 12.

At this time, the TFT biosensor 401 is driven by the silicon substrate11 that becomes the first gate electrode, and the voltage of thereference electrode 17 is kept to be constant. According to this, it ispossible to detect the redox potential, which is caused by reaction ofthe enzyme 19 with a substrate, as the Vth shift of Vref-Idcharacteristics.

As is the case with Example 3, when the enzyme 19 is intermittentlydisposed on the ion-sensitive insulating film 14, it is possible toincrease an effective surface area of the enzyme 19, and contact betweenthe ion-sensitive insulating film 14 and the sensing object material 16is secured. As a result, a sensitivity improving effect is obtained.

Example 4

Next, Example 4, which further specifies the third embodiment, isdescribed using FIG. 6.

In the TFT biosensor 401 of Example 4, the thermal oxide film 10 isformed on the silicon substrate 11 in the film thickness of 200 nm. Asilicon oxide film, a silicon nitride film, and the like, which areformed by plasma CVD method or sputtering method, may be used instead ofthe thermal oxide film 10.

In addition, the oxide semiconductor film composed of In—Ga—Zn—O andhaving the film thickness of 50 nm is formed on the silicon substrate 11covered with the thermal oxide film 10 by sputtering method using metalmask. In the film formation, the sintered In—Ga—Zn—O target is used, thesubstrate is not heated, and DC sputtering method in a mixed gasatmosphere of argon gas and oxygen gas is employed. After the filmformation, the annealing process is performed in the air at 400° C. forone hour. The semiconductor active layer 12 having an island shape isformed by patterning the oxide semiconductor film.

Continuously, the aluminum metal, or the aluminum metal that contains 1%of silicon is DC-sputtered by using metal mask in order to form thesource electrode 13 s and the drain electrode 13 d. The film thicknessof the source electrode 13 s and the drain electrode 13 d is set at 50nm, respectively. In addition, the second gate insulating film 24composed of aluminum oxide and having the film thickness of 200 nm isformed by sputtering method using metal mask. In the film formation,when a ratio of the argon gas and oxygen gas is appropriatelycontrolled, it is possible to use both a sintered body target composedof Al—O, and a metallic aluminum target. The substrate is not heated,and RF sputtering method is employed.

Then, the second gate electrode 25 composed of tungsten (w) metal andhaving the film thickness of 50 nm is formed by sputtering method usingmetal mask. In addition, the annealing process is performed in the airat 300° C. for one hour, and the protective insulating film 15 coversthe second gate electrode 25 except for a part thereof. Furthermore, inthe second gate electrode 25, a predetermined region (refer to a rightside in the drawing), which is opposite to a site that overlaps with thesemiconductor active layer 12, is exposed, and a region other than thepredetermined region is covered with the protective insulating film 15.It is preferable that a silicone resin is used as the protectiveinsulating film 15, but a photoresist, an epoxy resin, and the like maybe used as long as appropriate water resistance and insulatingproperties are obtained.

Next, the manufacturing apparatus adds a glucose dehydrogenase aqueoussolution dropwise to the region of the second gate electrode 25 which isnot covered with the protective insulating film 15, and dries theaqueous solution at room temperature for solidification. According tothis, the enzyme 19 is fixed onto the second gate electrode 25. In thisexample, the glucose dehydrogenase is used as the enzyme 19. However,there is no limitation thereto, and it is possible to employ othercombinations of enzyme and a substrate as long as a redox reactionprogresses on the second gate electrode 25.

In addition, the enzyme 19 is also not limited to a so-called enzyme,and a reaction between biomolecules, through which potential variationoccurs on the second gate electrode 25, may be applied as the sensingobject material 16 and the enzyme 19 in this example. For example,application can be expanded to antigen-antibody reaction, coupling oflectin and physiological active sugar chain, a mutual operation ofDNA-DNA or RNA-RNA, and coupling between inorganic compounds.

In the configuration of Example 4, the region over the TFT channel iscovered with the protective insulating film 15, and thus it is possibleto suppress intrusion of the sensing object material 16 (a test liquid,and the like) to the channel portion. As a result, reliability isimproved.

Fourth Embodiment

FIG. 7 is a cross-sectional view illustrating a TFT biosensor 501 offourth embodiment.

The TFT biosensor 501 of the fourth embodiment includes thesemiconductor active layer 12 to which the source electrode 13 s and thedrain electrode 13 d are connected. The thermal oxide film 10 as thefirst gate insulating film and the silicon substrate 11 as the firstgate electrode are provided on one surface (a first surface, a lowersurface in FIG. 7) of the semiconductor active layer 12. The second gateinsulating film 24, the second gate electrode 25, and the ion-sensitiveinsulating film 14 are provided on the other surface (a second surface,an upper surface in FIG. 7) of the semiconductor active layer 12. Theelectrostatic capacity per unit area of the second gate insulating film24 is set to be greater than the electrostatic capacity per unit area ofthe first gate insulating film (the thermal oxide film 10).

The second gate electrode 25 includes a region, which overlaps with thesemiconductor active layer 12, on the upper surface of the second gateinsulating film 24, and is disposed to two-dimensionally (to a rightdirection in FIG. 7) extend from the region. That is, the second gateelectrode 25 is provided on the second gate insulating film 24, andextends from the region, which overlaps the semiconductor active layer12, to a position that is two-dimensionally separated from the region.

The ion-sensitive insulating film 14 is provided on the upper surface ofthe second gate electrode 25. In addition, in the ion-sensitiveinsulating film 14, the enzyme 19 is fixed to the upper surface in aregion that is opposite to the region that overlaps with thesemiconductor active layer 12, and a region other than the region, inwhich the enzyme 19 is fixed, is covered with the protective insulatingfilm 15. The TFT biosensor 501 includes the reference electrode 17 atthe spatially separated position from the enzyme 19 that is fixed ontothe ion-sensitive insulating film 14, and the protective insulating film15. The sensing object material 16 is disposed on the ion-sensitiveinsulating film 14 and the enzyme 19, and the voltage from the referenceelectrode 17 is applied to the second gate electrode 25. The potentialvariation, which occurs through the reaction of enzyme 19, issuperimposed on the potential of the reference electrode 17, thispotential is transmitted to the second gate electrode 25, and induces atop channel in the semiconductor active layer 12 through the second gateinsulating film 24.

At this time, the TFT biosensor 501 is driven by the silicon substrate11 that becomes the first gate electrode, and the potential of thereference electrode 17 is kept to be constant. According to this, it ispossible to detect the potential, which is caused by the reaction ofenzyme 19 with the substrate, as the Vth shift of the Vref-Idcharacteristics.

Example 5

Next, Example 5, which further specifies the fourth embodiment, isdescribed using FIG. 7.

In the TFT biosensor 501 of Example 5, the semiconductor active layer12, the source electrode 13 s and drain electrode 13 d, the second gateinsulating film 24, the second gate electrode 25, and the ion-sensitiveinsulating film 14 are formed in this order on the silicon substrate 11covered with the thermal oxide film 10 by sputtering method using metalmask. The semiconductor active layer 12 is composed of In—Ga—Zn—O andhas the film thickness of 50 nm. The source electrode 13 s and the drainelectrode 13 d are composed of molybdenum metal and have the filmthickness of 100 nm. The second gate insulating film 24 is composed oftantalum oxide and has the film thickness of 100 nm. The second gateelectrode 25 is composed of molybdenum metal and has the film thicknessof 50 nm. The ion-sensitive insulating film 14 is composed of siliconoxide and has the film thickness of 100 nm.

At this time, the materials of the respective layers are not limited tothe above-described materials, and titanium (Ti), aluminum (Al),tungsten (w), tantalum (Ta), chromium (Cr), and an alloy film thereof,or a stacked film thereof can be used as the electrodes. In addition,aluminum oxide, silicon nitride (Si₃N₄), zirconium oxide (ZrO₂), hafniumoxide, strontium titanate (SrTiO₃), barium titanate (BaTiO₃), and astacked film thereof can be used as the insulating films.

In addition, the annealing process is performed in the air at 300° C.for one hour, and the protective insulating film 15 composed of siliconeresin covers the ion-sensitive insulating film 14 except for a partthereof. In addition, in the ion-sensitive insulating film 14, apredetermined region (refer to a right side in the drawing), which isopposite to a site that overlaps with the semiconductor active layer 12,is exposed, and a region other than the predetermined region is coveredwith the protective insulating film 15.

Next, the manufacturing apparatus adds the phosphate buffer solution setto pH 6.8 in which galectin as lectin that recognizes galactose in aspecific manner is dissolved, dropwise to the region of theion-sensitive insulating film 14 which is not covered with theprotective insulating film 15, and dries the phosphate buffer solutionin room temperature for solidification. According to this, the enzyme 19is fixed to the ion-sensitive insulating film 14. The enzyme 19 iscoupled to galactose in the specific manner, and allows the interfacialpotential of the ion-sensitive insulating film 14 to vary.

It is possible to measure galactose by detecting the potential variationwith the TFT sensor. Here, galectin is used as lectin, but there is nolimitation thereto. When using lectin having differentsubstrate-specificity, it is possible to provide a TFT biosensor inwhich a different physiological active sugar chain is set as an object.

In example 5, the ion-sensitive insulating film 14 is further providedbetween the enzyme 19 and the second gate electrode 25, compared withExample 4. When the insulating film 14 exists, it is possible tosuppress electrical short-circuiting between the sensing object material16 and the second gate electrode 25, and thus it is possible to furtherimprove reliability.

Example 6

Next, Example 6, which is a modification example of the fourthembodiment, is described using FIG. 8. FIG. 8 is a cross-sectional viewillustrating a TFT biosensor 601 of Example 6.

As is the case with Example 5, the TFT biosensor 601 of Example 6 hasthe structure in which the semiconductor active layer 12, the sourceelectrode 13 s and the drain electrode 13 d, the second gate insulatingfilm 24, the second gate electrode 25, and the ion-sensitive insulatingfilm 14 are stacked on the silicon substrate 11 on which the thermaloxide film 10 is formed. As manufacturing means, a sputtering methodthat uses a metal mask may be applied, or a photolithography method maybe used.

Next, the photoresist is patterned on the ion-sensitive insulating film14. As the photoresist that is used at this time, a lift-off dedicatedresist is preferable, but any resist may be used as long as the resistcan be easily removed with an organic solvent such as acetone. Inaddition, the manufacturing apparatus applies a phosphate buffersolution set to pH 6.8, in which alcohol dehydrogenase is dissolved,onto the photoresist that is patterned. As an application method, spincoating, dipping, and potting may be selected in accordance withviscosity of the solution. Then, the manufacturing apparatus removes thephotoresist, which is patterned, with acetone to form the enzyme 19 thatis patterned. The enzyme 19 is provided in a predetermined region of theion-sensitive insulating film 14 with a regular interval or in a randommanner.

Continuously, the protective insulating film 15 composed of siliconeresin covers the ion-sensitive insulating film 14 except for a region inwhich the enzyme 19 is patterned. Through the above-described processes,it is possible to provide the TFT biosensor 601 in which the TFT is usedas the interfacial potential detection mechanism and alcoholdehydrogenase is used as the biomolecule recognition mechanism, andwhich is capable of measuring an alcohol concentration.

Compared with Example 5, the enzyme 19 is intermittently disposed on theion-sensitive insulating film 14, and thus it is possible to increasethe effective surface area of the enzyme 19, and the contact between theion-sensitive insulating film 14 and the sensing object material 16 issecured in Example 6 as same as in Example 3. As a result, a sensitivityimproving effect is obtained.

Fifth Embodiment

FIG. 9 is a cross-sectional view illustrating a TFT biosensor 701 offifth embodiment.

The TFT biosensor 701 of the fifth embodiment includes the semiconductoractive layer 12 to which the source electrode 13 s and the drainelectrode 13 d are connected. The thermal oxide film 10 as the firstgate insulating film and the silicon substrate 11 as the first gateelectrode are provided on one surface (a first surface, a lower surfacein FIG. 9) of the semiconductor active layer 12. The second gateinsulating film 24, the second gate electrode 25, and the ion-sensitiveinsulating film 14 are provided on the other surface (a second surface,an upper surface in FIG. 9) of the semiconductor active layer 12.

The electrostatic capacity per unit area of the second gate insulatingfilm 24 is set to be greater than the electrostatic capacity per unitarea of the first gate insulating film (thermal oxide film 10).Furthermore, the position of the second gate electrode 25 and theion-sensitive insulating film 14 with respect to the upper surface ofthe second gate insulating film 24 is the same as in Examples 5 and 6.In addition, in the ion-sensitive insulating film 14, the predeterminedregion (refer to a right side in the drawing), which is opposite to aregion that overlaps with the semiconductor active layer 12, is exposed,and a region other than the predetermined region is covered with theprotective insulating film 15.

In addition, the TFT biosensor 701 of the fifth embodiment includes theenzyme 19 fixed to a second insulating substrate 18 at a positionspatially separated from the ion-sensitive insulating film 14. Inaddition, the TFT biosensor 701 includes the reference electrode 17 at aspace between the ion-sensitive insulating film 14 and the enzyme 19. Inthe TFT biosensor 701 of the fifth embodiment, in a case where thesensing object material 16 is disposed on the ion-sensitive insulatingfilm 14, the enzyme 19 reacts with the sensing object material 16, andcauses the pH variation in the vicinity. It is possible to measure theconcentration of the sensing object material 16 by grasping the pHvariation as potential variation on the surface of the ion-sensitiveinsulating film 14.

Example 7

Next, Example 7, which further specifies the fifth embodiment, isdescribed using FIG. 9.

In the TFT biosensor 701 of Example 7, the semiconductor active layer12, the source electrode 13 s and drain electrode 13 d, the second gateinsulating film 24, the second gate electrode 25, and the ion-sensitiveinsulating film 14 are formed on the silicon substrate 11 covered withthe thermal oxide film 10 by sputtering method, and are patterned byphotolithography method, respectively. The semiconductor active layer 12is composed of In—Ga—Zn—O and has the film thickness of 30 nm. Thesource electrode 13 s and the drain electrode 13 d are composed ofmolybdenum metal and have the film thickness of 50 nm, respectively. Thesecond gate insulating film 24 is composed of tantalum oxide and has thefilm thickness of 100 nm. The second gate electrode 25 is composed ofmolybdenum metal and has the film thickness of 50 nm. The ion-sensitiveinsulating film 14 is composed of silicon oxide and has the filmthickness of 50 nm.

Continuously, the annealing process is performed in the air at 300° C.for one hour, and the protective insulating film 15 composed of siliconeresin covers the ion-sensitive insulating film 14 except for a partthereof.

Then, the manufacturing apparatus fixes urease that becomes the enzyme19 on the second insulating substrate 18. As fixing means, as describedabove, the method such as spin coating, dipping, and potting of anenzyme aqueous solution can be used. The urease is enzyme thathydrolyzes urea to generate ammonia and carbon dioxide, and pH in thevicinity of the urease varies toward an alkaline side due to generationof ammonia. It is possible to measure the urea concentration by graspingthe variation with the TFT sensor. As described above, it is possible toprovide the TFT biosensor 701 capable of measuring the ureaconcentration.

As is the case with the TFT biosensor 101 of Example 1, the biosensor701 of Example 7 includes the biomolecule recognition mechanism (enzyme19) at the spatially separated position from the ion-sensitiveinsulating film 14. Accordingly, it is possible to suppress the mutualfunction interference of the pH measurement unit and the biologicalmaterial recognition mechanism. In addition, in a case where the enzyme19 is inactivated, it is possible to replace a substrate of the enzyme19 with new one while keeping a portion other than the substrate (thesecond insulating substrate 18) of the enzyme 19.

Even in Examples 5 to 7, as is the case with the TFT biosensor 401 ofExample 4, the TFT biosensors 501 to 701 are driven by the siliconsubstrate 11 that becomes the first gate electrode, and the voltage ofthe reference electrode 17 is kept to be constant. According to this, itis possible to detect the redox potential, which is caused by thereaction of enzyme 19 with the substrate, as the Vth shift of Vref-Idcharacteristics. In addition, even in Examples 5 to 7, the TFTbiosensors 501 to 701 are provided with a detection unit that detectspotential difference, which occurs between the ion-sensitive insulatingfilm 14 and the sensing object material 16, after amplifying thepotential difference by multiplying the potential difference by thevalue of the ratio obtained by dividing the electrostatic capacity perunit area of the second gate insulating film 24 by the electrostaticcapacity per unit area of the first gate insulating film (thermal oxidefilm 10). According to this, it is possible to realize the biosensorhaving high sensitivity.

Sixth Embodiment

A sixth embodiment is described using FIGS. 10A and 10B, and FIG. 11.FIGS. 10A and 10B are cross-sectional views illustrating a TFT biosensorof the sixth embodiment, and FIG. 11 is a schematic view thereof.

As illustrated in FIG. 10A, in the TFT biosensor of the sixthembodiment, a thin film transistor, which includes the first gateelectrode 22, the first gate insulating film 23, the semiconductoractive layer 12, the source electrode 13 s and drain electrode 13 d, theion-sensitive insulating film 14, and the protective insulating film 15,is formed on a first insulating substrate 26. In addition, in the TFTbiosensor of the sixth embodiment, the reference electrode 17 isdisposed at the spatially separated position from the ion-sensitiveinsulating film 14. In an example illustrated in FIG. 10A, the referenceelectrode 17 is disposed at the spatially separated position from theprotective insulating film 15, but there is no limitation to thedisposition. As illustrated in FIG. 10B, the reference electrode 17 maybe formed on the protective insulating film 15. In this case, a silverthin film is formed on the protective insulating film 15, and isimmersed in hydrochloric acid and the like to form a silver chloridefilm on a surface of the silver thin film. Then, a stacked thin film ofsilver chloride/silver is patterned to a desired shape to form thereference electrode 17. A configuration, which includes the thin filmtransistor and the reference electrode 17, is referred to as a thin filmtransistor sensor unit S. An example of the thin film transistor sensorunit S is the TFT biosensor 101 in FIG. 1, the TFT biosensor 201 inFIGS. 3 and 4, and the TFT biosensor 301 in FIG. 5.

In addition, in the sixth embodiment, the enzyme 19 havingsubstrate-specificity for the biological material is formed on thesecond insulating substrate 18. The enzyme 19 may be formed on theentirety of the second insulating substrate 18. In addition, a groovemay be formed in the second insulating substrate 18 in advance, and theenzyme 19 may be selectively formed at the groove portion. In a casewhere the enzyme 19 is formed on the entirety of the second insulatingsubstrate 18, the first insulating substrate 26 and the secondinsulating substrate 18 are fixed in a state in which the ion-sensitiveinsulating film 14 and the enzyme 19 face each other and a space issecured therebetween. This space becomes a flow path P through which thesolution that includes the sensing object material 16 flows.

In addition, the configuration, in which the groove is formed in thesecond insulating substrate 18, is illustrated in FIG. 11. In an exampleillustrated in FIG. 11, the groove 18 a having a predetermined width isformed at an appropriate site in one surface of the second insulatingsubstrate 18 (in FIG. 11, the central portion of a lower surface in ahorizontal direction), and the enzyme 19 is formed on an inner side ofgroove 18 a. In a case where the groove 18 a is formed in the secondinsulating substrate 18, it is possible to closely join the firstinsulating substrate 26 and the second insulating substrate 18. When thetwo insulating substrates 26 and 18 are joined, a space is formed due tothe groove 18 a, and this space becomes the flow path P through whichthe solution that includes the sensing object material 16 flows. It isnecessary for the insulating substrates 18 and 26 to be joined in astate in which the groove 18 a and the thin film transistor sensor unitS two-dimensionally overlap each other.

Further, in the TFT biosensor of the sixth embodiment, a mechanism (forexample, a pump) M, which supplies the solution that includes thesensing object material 16 to the flow path P and controls the flow ofthe sensing object material 16, may be provided regardless of whether ornot the groove 18 a exists. For example, as illustrated in FIG. 11, thesensing object material 16 is supplied by the mechanism M from one endside of the flow path P as indicated by an arrow M1. The sensing objectmaterial 16 passes through a portion on the thin film transistor sensorunit S in the flow path P, and is discharged from the other end side ofthe flow path P as indicated by an arrow M2. Even in the exampleillustrated in FIGS. 10A and 10B, for example, the sensing objectmaterial 16 may be allowed to pass through the flow path P by themechanism M from a left side of the thin film transistor sensor unit Sto a right side thereof.

FIG. 10B illustrates a case where the enzyme 19 is also formed on thefirst insulating substrate 26. As is the case with the abovedescription, the thin film transistor is formed on the first insulatingsubstrate 26, and then the enzyme 19 is formed in a desired region otherthan the thin film transistor region. Although not illustrated in thedrawing, it is also possible to employ a configuration in which theenzyme 19 is formed only in a region other than the thin film transistorregion on the first insulating substrate 26, and the enzyme 19 is notformed on the second insulating substrate 18.

In the sixth embodiment, the enzyme 19 is formed on the secondinsulating substrate 18, or the region on the first insulating substrate26 other than the thin film transistor region, that is, on a wide areaat the spatially separated position from the pH sensing unit (thin filmtransistor sensor unit S). Accordingly, two functions including afunction of the pH sensing unit and biomolecule recognition function donot inhibit each other. In addition, it is possible to increase theamount of enzyme per unit area in the pH sensing unit, and thus it ispossible to enlarge the amount of pH variation due to enzyme reaction.As a result, it is possible to raise detection sensitivity for thebiological material.

Furthermore, although not illustrated in FIGS. 10A and 10B, and FIG. 11,as illustrated in FIG. 1, the TFT biosensor of the sixth embodimentincludes any one of the voltage detection unit 20 that reads out thepotential difference between the source electrode 13 s and the gateelectrode 22, and the current detection unit 21 that reads out thecurrent that flows to the source electrode 13 s or the drain electrode13 d.

Example 8

Example 8 of the sixth embodiment is described using FIG. 10A.

In the TFT biosensor of Example 8, an aluminum alloy film is formed on aglass substrate that is the first insulating substrate 26 by sputteringmethod, and is patterned in a desired shape in order to form the firstgate electrode 22. Then, the silicon oxide film having the filmthickness of 200 nm is formed by plasma CVD method as the first gateinsulating film 23.

In addition, the In—Ga—Zn—O film is formed as the semiconductor activelayer 12 by sputtering method, and is patterned in a desired shape.After performing annealing in the air at 400° C. for one hour, themolybdenum film is formed by sputtering method, and is patterned in adesired shape to form the source electrode 13 s and the drain electrode13 d. Continuously, the tantalum oxide film having the film thickness of100 nm is formed as the ion-sensitive insulating film 14 by sputteringmethod, and is patterned in a desired shape.

In addition, the annealing process is performed in the air at 300° C.for one hour, and the protective insulating film 15 is formed in thedesired shape by using silicone resin. When considering that a specificdielectric constant of silicon oxide is 4, and a specific dielectricconstant of tantalum oxide is 20, in this configuration, theelectrostatic capacity per unit area of the ion-sensitive insulatingfilm 14 is approximately 10 times larger than the electrostatic capacityper unit area of the first gate insulating film 23. According to this,it is possible to realize pH detection sensitivity of approximately 10times larger than the theoretical limit of the Nernst theory.

Next, the manufacturing apparatus fixes glucose oxidase as the enzyme 19onto the glass substrate that is the second insulating substrate 18.Specifically, a reagent, which is obtained by dissolving 10% glucoseoxidase, 10% bovine serum albumin, and 8% glutaraldehyde in a phosphatebuffer, is prepared, and is used as the enzyme 19. The manufacturingapparatus applies the enzyme 19 onto the glass substrate, patterns theenzyme 19 to a desired shape, and dries the enzyme 19 at roomtemperature for 20 minutes, thereby fixing the enzyme 19 onto the secondinsulating substrate 18.

The manufacturing apparatus sets the two sheets of glass substrates(insulating substrates 26 and 18), which are manufactured as describedabove, to face each other so that the tantalum oxide film (ion-sensitiveinsulating film 14) and the glucose oxidase (enzyme 19) face each other,and seals peripheral edge portions of the two sheets of glass substrates26 and 18 in a state in which a gap of several mm from several 100 μm tois opened as the flow path P. In addition, the reference electrode 17,which is formed from silver/silver chloride, is inserted into the gap.As a mechanism M that introduces a test liquid (solution that includesthe sensing object material 16) into the gap (flow path P), for example,a micropump is provided.

As the test liquid, a glucose aqueous solution, which has variousconcentrations, is introduced into the gap (flow path P) by using themicropump, and a minute variation in a proton concentration, which iscaused by new generation of protons through the enzyme reaction betweenglucose and glucose oxidase (enzyme 19), is detected from the pHvariation of the test liquid. In this example, the glucose oxidase thatis the enzyme 19 is fixed to a wide region on the surface of the glasssubstrate (second insulating substrate 18), and thus an area, in whichthe glucose oxidase comes into contact with the glucose aqueous solutionthat is the test liquid, is sufficiently great. Accordingly, the enzymereaction efficiently progresses, and a variation in a glucoseconcentration of approximately 0.001 mM can be detected in combinationwith high sensitivity in pH sensing by using a top gate effect.

Example 9

Example 9 of the sixth embodiment is described using FIG. 11. As is thecase with Example 8 as described above, a manufacturing apparatus inExample 9 forms the thin film transistor sensor unit S on the glasssubstrate that is the first insulating substrate 26. Wirings of thefirst gate electrode 22, the source electrode 13 s, the drain electrode13 d, and the reference electrode 17 of the thin film transistor sensorunit S are lead out to the outside of the glass substrate so as to applyan operation electrical signal from the outside. In addition, themanufacturing apparatus forms the groove 18 a in the glass substratethat is the second insulating substrate 18. For example, the glass isetched with fluoric acid to form the groove 18 a having a width of 2 mmand a depth of 800 μm.

Then, as is the case with Example 8, the manufacturing apparatus fixesthe glucose oxidase (enzyme 19) to the concave surface (inner surface)of the groove 18 a. In addition, the manufacturing apparatus joins twosheets of glass substrates (insulating substrate 26 and 18) in anarrangement in which the thin film transistor sensor unit S and thegroove 18 a two-dimensionally overlap each other. That is, in a state inwhich the thin film transistor sensor unit S is covered with the groove18 a, the two insulating substrates 26 and 18 are joined to each other.

With respect to this device, as the test liquid, a glucose solution,which has various concentrations, is introduced into the flow path Pfrom one side of the flow path P formed by the groove 18 a by using themicropump M, and the test liquid is discharged from the other side ofthe flow path P. When the test liquid flows at the inside of the flowpath P, the test liquid reacts with the enzyme 19 that is fixed at theinside of the flow path P, thereby generating protons. It is possible todetect a minute glucose concentration by detecting a generation amountof protons as the variation in the electric double-layer potential, byusing the thin film transistor sensor unit S. With regard to thestructure of the flow path P, it is possible to employ a structure, inwhich a wide-width region is partially configured partway through theflow path P to increase an area capable of fixing the enzyme 19, withoutlimitation to a linear structure as illustrated in FIG. 11.

Seventh Embodiment

The description will be given of seventh embodiment with reference toFIGS. 12A and 12B. FIGS. 12A and 12B are cross-sectional viewsillustrating a TFT biosensor of the seventh embodiment. As illustratedin FIG. 12A, in the TFT biosensor of the seventh embodiment, the thinfilm transistor, which includes the first gate electrode 22, the firstgate insulating film 23, the semiconductor active layer 12, the sourceelectrode 13 s and drain electrode 13 d, the second gate insulating film24, the second gate electrode 25, the ion-sensitive insulating film 14,and the protective insulating film 15, is formed on the first insulatingsubstrate 26. The second gate electrode 25 has a shape that extends froma position, which faces the first gate electrode 22, in a horizontaldirection, and the ion-sensitive insulating film 14 is formed over theextension region. An opening is formed in a part of a region, whichextends in the horizontal direction, on the ion-sensitive insulatingfilm 14, and the protective insulating film 15 is formed in a regionother than the opening. That is, the channel portion and ion-sensitiveportion of the thin film transistor are disposed at positions whichtwo-dimensionally deviate from each other. This configuration isdifferent from the configuration of FIGS. 10A, 10B, and 11 in which thechannel portion and the ion-sensitive portion are disposed at positionswhich two-dimensionally overlap each other.

In addition, in the TFT biosensor of the seventh embodiment, thereference electrode 17 is disposed at the spatially separated positionfrom the ion-sensitive insulating film 14. In FIG. 12A, the referenceelectrode 17 is disposed at the spatially separated position from theprotective insulating film 15, but the reference electrode 17 may beformed on the protective insulating film 15 as illustrated in FIG. 12Bwithout limitation to the above-described disposition. In this case, asilver thin film is formed on the protective insulating film 15, and isimmersed in hydrochloric acid and the like to form a silver chloridefilm on a surface of the silver thin film. Then, a stacked thin film ofsilver chloride/silver is patterned to a desired shape to form thereference electrode 17. In this example, the configuration including thethin film transistor and the reference electrode 17 is also referred toas the thin film transistor sensor unit S. An example of the thin filmtransistor sensor unit S is the TFT biosensor 401 in FIG. 6, the TFTbiosensor 501 in FIG. 7, the TFT biosensor 601 in FIG. 8, and the TFTbiosensor 701 in FIG. 9.

In addition, in the seventh embodiment, the enzyme 19 havingsubstrate-specificity for the biological material is formed on thesecond insulating substrate 18. The enzyme 19 may be formed on theentirety of the second insulating substrate 18. In addition, afterforming the groove 18 a in the second insulating substrate 18 in advancesimilar to FIG. 11, the enzyme 19 may be selectively formed in theportion of the groove 18 a. The first and second insulating substrates26 and 18 are fixed in a state in which the ion-sensitive insulatingfilm 14 and the enzyme 19 face each other, and the space is securedtherebetween. The space becomes the flow path P through which thesensing object material 16 flows. In addition, in the case of formingthe groove 18 a in the second insulating substrate 18, it is possible toclosely join the first and second insulating substrates 26 and 18. Inthis case, it is necessary to perform the joining so that the groove 18a and the thin film transistor sensor unit S two-dimensionally overlapeach other. Further, the mechanism (for example, a pump and the like) M,which supplies the sensing object material 16 to the flow path P andcontrols the flow of the sensing object material 16, may be providedregardless of whether or not the groove 18 a exists.

FIG. 12B illustrates a case where the enzyme 19 is also formed on thefirst insulating substrate 26. As described above, after forming thethin film transistor on the first insulating substrate 26, the enzyme 19is formed in a desired region other than the thin film transistorregion. Although not illustrated in the drawing, it is also possible toemploy a configuration in which the enzyme 19 is formed only in a regionother than the thin film transistor region on the first insulatingsubstrate 26, and the enzyme is not formed on the second insulatingsubstrate 18.

In the seventh embodiment, the enzyme 19 is formed on the secondinsulating substrate 18, or a region on the first insulating substrate26 other than the thin film transistor region, that is, on a wide areaat the spatially separated position from the pH sensing unit (thin filmtransistor sensor unit S). Accordingly, two functions including functionof the pH sensing unit and biomolecule recognition function do notinhibit each other. In addition, it is possible to increase the amountof enzyme per unit area of the pH sensing unit, and thus it is possibleto enlarge the variation amount of pH due to enzyme reaction. As aresult, it is possible to raise detection sensitivity for the biologicalmaterial. In addition, compared with the sixth embodiment, in theseventh embodiment, the ion-sensitive insulating film 14 exists at theposition that is spaced away from the thin film transistor region. Theion-sensitive insulating film 14 is a portion with which a test liquidcomes into contact, and in this example in which the portion is spacedaway from the thin film transistor region, a probability that the testliquid penetrates into the thin film transistor sensor unit S decreases,and thus it is possible to expect long lifetime of the sensor element.

In addition, FIGS. 12A and 12B illustrate a configuration in which theion-sensitive insulating film 14 is provided on the second gateelectrode 25. However, in a case where the second gate electrode 25itself has ion sensitiveness, the ion-sensitive insulating film 14 maynot be provided.

Example 10

Example 10 of the seventh embodiment is described using FIG. 12A.

In the TFT biosensor of Example 10, the aluminum alloy film is formed onthe glass substrate that is the first insulating substrate 26 bysputtering method, and is patterned in the desired shape in order toform the first gate electrode 22. Then, the silicon oxide film havingthe film thickness of 200 nm is formed as the first gate insulating film23 by plasma CVD method.

In addition, the In—Ga—Zn—O film is formed as the semiconductor activelayer 12 by sputtering method, and is patterned in the desired shape.After performing annealing in the air at 400° C. for one hour, themolybdenum alloy film is formed by sputtering method, and is patternedin the desired shape to form the source electrode 13 s and the drainelectrode 13 d. Continuously, the tantalum oxide film having the filmthickness of 100 nm is formed as the second gate insulating film 24 bysputtering method, and is patterned in the desired shape.

Next, after performing annealing in the air at 300° C. for one hour, thealuminum alloy film is formed as the second gate electrode 25, and ispatterned in the desired shape. At this time, the second gate electrode25 is set to have a shape that extends from the position that faces thefirst gate electrode 22 in a horizontal direction. In addition, thetantalum oxide film having the film thickness of 10 nm is formed as theion-sensitive insulating film 14 on the second gate electrode 25, and ispatterned in the desired shape.

Continuously, the protective insulating film 15 is formed in a desiredshape by using epoxy resin. At this time, patterning is performed sothat an opening is formed in a part of a region, which extends in thehorizontal direction, on the ion-sensitive insulating film 14. Whenconsidering that the specific dielectric constant of the silicon oxideis 4, and the specific dielectric constant of the tantalum oxide is 20,in this configuration, the electrostatic capacity per unit area of theion-sensitive insulating film 14 is approximately nine times larger thanthe electrostatic capacity per unit area of the first gate insulatingfilm 23. According to this, it is possible to realize pH detectionsensitivity of approximately nine times larger than the theoreticallimit of the Nernst theory. The reason why the sensitivity slightlydecreases compared with the case of Example 8 is as follows. In thisexample, it is necessary consider series connection between the tantalumoxide (second gate insulating film 24) having the film thickness of 100nm and the tantalum oxide (ion-sensitive insulating film 14) having thefilm thickness of 10 nm as a capacity.

Next, the manufacturing apparatus fixes penicillinase as the enzyme 19onto the glass substrate that is the second insulating substrate 18. Forexample, after preparing a reagent in which penicillinase is dissolvedin a buffer solution, the manufacturing apparatus applies the reagentonto the glass substrate (second insulating substrate 18), and patternsthe reagent to a desired shape. Then, the manufacturing apparatus driesthe reagent at room temperature for 120 minutes to fix the enzyme 19onto the second insulating substrate 18.

The manufacturing apparatus sets the two sheets of glass substrates toface each other so that the tantalum oxide film (ion-sensitiveinsulating film 14) and the penicillinase (enzyme 19) face each other,and seals the peripheral edge portion of the two sheets of glasssubstrates in a state in which a gap of several mm from several 100 μmis opened as the flow path P. In addition, the manufacturing apparatusinserts the reference electrode 17, which is formed from silver/silverchloride, into the gap. In addition, the manufacturing apparatusprovides the micropump as the mechanism M that introduces a test liquid(solution that includes the sensing object material 16) into the gap(flow path P).

In the TFT biosensor that is manufactured as described above, as thetest liquid, a penicillin aqueous solution, which has variousconcentrations, is introduced into the gap (flow path P) by using themicropump M, and a minute variation in a proton concentration, which iscaused by new generation of protons through the enzyme reaction betweenpenicillin and penicillinase (enzyme 19), is detected from the pHvariation of the test liquid. The penicillin is hydrolyzed through theenzyme reaction between penicillin and penicillinase, and thuspenicilloic acid and protons are generated. Particularly, in biosensorfield, detection of a minute penicillin concentration is important, andit is necessary to sense a minute variation in the proton concentrationwhich occurs by the enzyme reaction. In this example, the penicillinase,which is the enzyme 19, is fixed in a wide region on the surface of theglass substrate (second insulating substrate 18), and thus an area, inwhich the penicillin aqueous solution that is a test liquid and thepenicillinase come into contact with each other, is sufficiently great.

Accordingly, the enzyme reaction efficiently progresses, and a variationin the penicillin concentration of approximately 0.001 mM can bedetected in combination with high sensitivity in pH sensing by using atop gate effect. In addition, compared with the case of Example 8, inthis example, the ion-sensitive unit exists at the position that isspaced from the thin film transistor region, and thus a probability thatthe penicillin aqueous solution, which is the test liquid, penetratesinto the thin film transistor unit decreases. As a result, it ispossible to realize long lifetime and a high yield ratio of the sensorelement.

In Examples 1 to 7 as described above, the descriptions have been givenof a case where a silicon wafer equipped with the thermal oxide film 10is used, and as constituent elements of the thin film transistor, thesilicon substrate 11 is allowed to operate as the gate electrode, andthe thermal oxide film 10 is allowed to operate as the gate insulatingfilm. However, there is no limitation thereto, and even in Examples 1 to7, as is the case with Examples 8 to 10, the gate electrode may beformed with the metal on the glass substrate (first insulating substrate26), and the gate insulating film (for example, a silicon oxide film,and the like) may be formed on the gate electrode by plasma CVD methodor sputtering method. In addition, even in the case of Examples 1 to 7,the groove 18 a may be formed in the second insulating substrate 18similar to FIG. 11, and the enzyme 19 a may be provided in the groove 18a.

In addition, in Examples 8 to 10, the enzyme 19 is fixed to thesubstrate (second insulating substrate 18) that is different from thesubstrate on the TFT sensor unit S side. Therefore, when the enzyme 19is inactivated, it is possible to recover the sensor function byreplacing only the enzyme substrate with new one while keeping theconfiguration other than the enzyme substrate (second insulatingsubstrate 18). Accordingly, it is possible to provide the sensor havinglong lifetime with the small burden on a user. In addition, the functioncapable of replacing the enzyme substrate with new one represents thatsubstitution with another enzyme is possible. That is, when consideringthe examples, for example, in a case where the substrate for glucosesensing and the substrate for penicillin sensing are provided as theenzyme substrate, it is possible to provide the TFT biosensor capable ofrealizing multi-item measurement by using the same TFT sensor substrate(first insulating substrate 26).

In addition, in the above-described examples, the descriptions have beengiven of a case where the first insulating substrate 26 and the secondinsulating substrate 18 are disposed to face each other or are joined toeach other. However, there is no limitation thereto, and for example,the first insulating substrate 26, on which the thin film transistorsensor unit S is formed, may have a flat shape, and the secondinsulating substrate 18, to which the enzyme 19 is fixed, may have acylindrical shape. In this case, the effect can be realized by aconfiguration in which the enzyme 19 is fixed to an inner side of thecylinder, and the first insulating substrate 26 is disposed in thecylinder. In this manner, the disposition of the first insulatingsubstrate 26 and second insulating substrate 18 may be set in anarbitrary manner.

In addition, in the above-described examples, illustration has beengiven of a case where the concentration (pH) of hydrogen ions varies dueto the enzyme reaction, and the concentration of biomaterial is sensedby detecting the pH variation. However, it is also possible to sense aconcentration of an arbitrary ion that is generated in the enzymereaction without limitation to the concentration of hydrogen ions. Forexample, in the case of performing sensing of the test liquid in which acation such as Na ion and K ion is generated through enzyme reaction,the following thin film is used (also referred to “applied”) as theion-sensitive insulating film 14. This thin film is a thin film that isformed by mixing a compound including polypeptide such as valinomycin orcrown ether that becomes a ligand as a basic skeleton, and a resinmaterial, and by applying and baking the resultant mixed material. Inthis case, the ion-sensitive film also operates as enzyme. Even in thecase of an anion without limitation to the cation, it is possible to usea ligand that is appropriate to the anion.

In addition, in the above-described examples, illustrated has been givenof the case of using the oxide semiconductor as the semiconductor activelayer 12. However, there is no limitation thereto, and it is alsopossible to use the following material (also referred to as a materialquality) as the semiconductor active layer 12. Examples of the materialinclude a silicon semiconductor such as amorphous silicon andpolycrystalline silicon, a low-molecular-weight-based organicsemiconductor (for example, pentacene and the like) that is capable ofbeing formed as a film through deposition, a high-molecular-weight-basedorganic semiconductor that is capable of being formed as a film throughapplication, and a carbon material such as carbon nanotube and graphene.As a specific example, FIG. 13 illustrates a configuration in which anorganic semiconductor is used as the active layer.

FIG. 13 is a cross-sectional view illustrating a TFT biosensor in whichan organic semiconductor is used as the semiconductor active layer 12.The TFT biosensor illustrated in FIG. 13 has the same configuration asthe TFT biosensor of Example 8 illustrated in FIG. 10A except that thesemiconductor active layer 12 is constituted by the organicsemiconductor. The oxide semiconductor frequently appears only n-typeconduction, and the organic semiconductor frequently appears only p-typeconduction. This kind of single polarity is easy to realize a top gateeffect, and is easy to realize high sensitivity in accordance with acapacity ratio.

Eighth Embodiment

FIG. 14 is a circuit diagram of a TFT biosensor device of eighthembodiment. FIG. 15 is a view illustrating a configuration example of amicroprocessor 38 in the TFT biosensor device in FIG. 14. The TFTbiosensor device (detection device) of the eighth embodiment includesany one of the TFT biosensors 101 to 701 of Examples 1 to 10 describedabove. The TFT biosensor in FIG. 14 is any one of the TFT biosensors 101to 701.

In addition, the TFT biosensor device includes a microprocessor(processor) 38, a constant voltage circuit (voltage application circuit)40, and a current-voltage conversion circuit (detection circuit) 41. Theconstant voltage circuit 40 applies a voltage (first voltage) betweenthe source electrode 13 s and drain electrode 13 d of the TFT biosensor.The current-voltage conversion circuit 41 detects a current, which flowsbetween the source electrode 13 s and drain electrode 13 d, as a voltage(second voltage). The microprocessor 38 controls a potential of the gateelectrode 13 g of the TFT biosensor and the constant voltage circuit 40based on the voltage that is detected by the current-voltage conversioncircuit 41.

The constant voltage circuit 40 includes a Zener diode 40 a and aresistor R1. The Zener diode 40 a is connected to a transistor 39through the resistor R1. The resistor R1 is connected to a power supplyVdd through the transistor 39. An output terminal of the constantvoltage circuit 40 is connected to the drain electrode 13 d of the TFTbiosensor through a resistor R2. In a case where the transistor 39 isturned on by the microprocessor 38, the constant voltage circuit 40applies a reverse breakdown voltage (V1) of the Zener diode 40 a betweenthe source electrode 13 s and the drain electrode 13 d of the TFTbiosensor through the resistor R2.

The current-voltage conversion circuit 41 converts a minute currentbetween the source electrode 13 s and the drain electrode 13 d into avoltage value. The current-voltage conversion circuit 41 includes afirst operational amplifier 41 a, a second operation amplifier 41 b, anda third operational amplifier 41 c.

Specifically, the current-voltage conversion circuit 41 includes thethree operational amplifiers 41 a to 41 c, and seven resistors R3 to R9.A positive input terminal (hereinafter, referred to as “+inputterminal”) of the first operational amplifier 41 a is connected to anoutput terminal of the constant voltage circuit 40, and a +inputterminal of the second operation amplifier 41 b is connected to thedrain electrode 13 d of the TFT biosensor. In addition, an outputterminal and a negative input terminal (hereinafter, referred to as“−input terminal”) of the first operational amplifier 41 a are connectedto each other through the resistor R4, and an output terminal and a−input terminal of the second operation amplifier 41 b are connected toeach other through the resistor R5. I addition, the −input terminal ofthe first operational amplifier 41 a and the −input terminal of thesecond operation amplifier 41 b are connected to each other through theresistor R3. A −input terminal of the third operational amplifier 41 cis connected to the output terminal of the first operational amplifier41 a through the resistor R6, and a +input terminal of the thirdoperational amplifier 41 c is connected to the output terminal of thesecond operation amplifier 41 b through the resistor R7. In addition,the +input terminal of the third operational amplifier 41 c is connectedto a ground through the resistor R9, and an output terminal and the−input terminal of the third operational amplifier 41 c are connected toeach other through the resistor R8. In addition, an output terminal ofthe third operational amplifier 41 c becomes an output terminal of thecurrent-voltage conversion circuit 41. The current-voltage conversioncircuit 41 outputs a voltage value (input voltage Vin), which isobtained by converting a minute current between the source electrode 13s and the drain electrode 13 d, to the microprocessor 38.

As illustrated in FIG. 15, the microprocessor 38 includes an operationunit 38 a and a storage unit 38 b. For example, the operation unit 38 ais constituted by one or a plurality of central processing units (CPUs),a multi-core CPU, and the like. For example, the storage unit 38 bincludes a random access memory (RAM) 38 ba, a read only memory (ROM) 38bb, and the like. The operation unit 38 a reads out a control program,which is stored in the ROM 38 bb into the RAM 38 ba, and executes thecontrol program so as to perform various operation processes. Forexample, when activating the microprocessor 38, the operation unit 38 areads out a control program file (execution file) for execution of afirst operation process from the ROM 38 bb, and develops and executesthe control program file in the RAM 38 ba, thereby functioning as aprogram module of a first operation unit 38 aa. In addition, theoperation unit 38 a reads out a control program file for execution of asecond operation process from the ROM 38 bb, and develops and executesthe control program file in the RAM 38 ba, thereby functioning as aprogram module of a second operation unit 38 ab.

The ROM 38 bb is a non-volatile memory, and stores a control program forexecution of a predetermined operation process by the operation unit 38a, and a control variable that is used when the operation unit 38 aperforms a predetermined operation process, various pieces of data, atable, and the like in advance. For example, in the case of allowing theoperation unit 38 a to perform an operation related to aproportional-integral-differential (PID) control, a control program forrealization of the PID control and a control variable that is used inthe PID control are stored in the ROM 38 bb. In addition, in the case ofallowing the operation unit 38 a to perform an operation related to aproportional-integral (PI) control, a control program for realization ofthe PI control and a control variable that is used in the PI control arestored in the ROM 38 bb. In addition, in the case of allowing theoperation unit 38 a to perform an operation related to a proportional(P) control, a control program for realization of the P control and acontrol variable that is used in the P control are stored in the ROM 38bb. The RAM 38 ba is a rewritable memory, and temporarily stores datathat is generated during an operation process by the operation unit 38a.

The microprocessor 38 acquires the voltage value, which is output fromthe current-voltage conversion circuit 41, as an input voltage Vin. Theoperation unit 38 a (first operation unit 38 aa) performs apredetermined operation (for example, the PID control, the PI control,and the P control) based on the input voltage Vin that is acquired fromthe current-voltage conversion circuit 41 and the control variable thatis stored in the ROM 38 bb to calculate the voltage value that isapplied to the gate electrode 13 g of the TFT biosensor. Furthermore,the first operation unit 38 aa calculates a voltage value (outputvoltage Vout1 that is applied to the gate electrode 13 g) so that thevoltage that is applied between the source electrode 13 s and the drainelectrode 13 d by the constant voltage circuit 40, and the voltagebetween the source electrode 13 s and the drain electrode 13 d, which isdetected by the current-voltage conversion circuit 41, become constant.The operation unit 38 a controls a voltage that is applied to the gateelectrode 13 g of the TFT biosensor based on the voltage value (outputvoltage Vout1) that is calculated. Specifically, the microprocessor 38applies the output voltage Vout1 of the voltage value, which iscalculated by the operation unit 38 a, to the gate electrode 13 g. Inaddition, the operation unit 38 a (second operation unit 38 ab)calculates an ion concentration, which corresponds to the output voltageVout1, based on the voltage value (output voltage Vout1) that iscalculated, and the table that is stored in the ROM 38 bb. Specifically,the second operation unit 38 ab reads out the ion concentration, whichcorresponds to the output voltage Vout1, from the table.

In addition, the operation unit 38 a applies a voltage (output voltageVout2) for turning ON or OFF of the transistor 39 to the transistor 39.

In the TFT biosensor device having the above-described configuration, inthe case of performing sensing by the TFT biosensor, the microprocessor38 sets the potential of the source electrode 13 s and the potential ofthe reference electrode 17 to the same potential, and controls thepotential between the silicon substrate 11 that becomes the gateelectrode 13 g and the source electrode 13 s so that a predeterminedcurrent I1 flows between the source electrode 13 s and the drainelectrode 13 d.

In the example illustrated in FIGS. 14 and 15, the microprocessor 38switches the potential of the source electrode 13 s to the sourcepotential or the reference potential and allows the potential of thesilicon substrate 11 that becomes the gate electrode 13 g to vary. Themicroprocessor 38 reads out potential difference, which is caused by theion concentration in the sensing object material 16, between the gateelectrode 13 g and the source electrode 13 s. In addition, a table(correspondence table), in which a correspondence between the ionconcentration and the potential difference (voltage value) between thegate electrode 13 g and the source electrode 13 s is stored as acharacteristics of the TFT biosensor, is stored in the storage unit 38 b(ROM 38 bb) in advance. The ion concentration, which corresponds to theread-out potential difference between the gate electrode 13 g and thesource electrode 13 s, in the sensing object material 16 is sensed(specified) with reference to the table.

The current-voltage conversion circuit 41 converts a minute currentbetween the source electrode 13 s and drain electrode 13 d into avoltage value, and applies the voltage value, which is converted, to themicroprocessor 38 as the input voltage Vin. The microprocessor 38controls the voltage (output voltage Vout1), which is applied to thegate electrode 13 g (silicon substrate 11) of the TFT biosensor so thatthe input voltage Vin becomes a constant value. The current-voltageconversion circuit 41 converts the drain-to-source current (Ids) of theTFT that depends on ion concentration in the object material into thevoltage signal. The microprocessor 38 acquires the voltage signal fromthe current-voltage conversion circuit 41 and outputs a voltage signal(Vg-s) to the gate electrode 13 g of the TFT so that the predeterminedcurrent (Ids) flows between the drain electrode 13 d and sourceelectrode 13 s regardless of ion concentration in the object material.In other words, the microprocessor 38 functions as a feedback circuit tokeep the current (Ids) constant regardless of ion concentration in theobject material, and thus the TFT biosensor (pH sensor) operates in aconstant-voltage (Vg-s) and constant-current (Ids) mode. In the case ofstopping the operation of the TFT biosensor, the microprocessor 38 doesnot output the output voltage Vout1, and in the case of operating theTFT biosensor, the microprocessor 38 outputs the output voltage Vout1.

In the eighth embodiment, the operation unit 38 a executes a controlprogram that is stored in the storage unit 38 b (ROM 38 bb) to realize aprocess in the microprocessor 38. In addition to this, a part or theentirety of the process, which is executed by the operation unit 38 a,may be realized by a dedicated hardware circuit.

Example 11

Example 11 of the eighth embodiment is described using FIGS. 14 and 15.Characteristics of the TFT biosensor temporally vary, and thus themicroprocessor 38 performs the following process. Specifically, whensensing an ion concentration, the microprocessor 38 sets the potentialof the source electrode 13 s, and each potential of the drain electrode13 d and the gate electrode 13 g (silicon substrate 11) to the samepotential. Then, the microprocessor 38 applies a predetermined potentialto the drain electrode 13 d and the gate electrode 13 g (siliconsubstrate 11). In other words, in the TFT biosensor, the potentialdifference between the source electrode 13 s and the drain electrode 13d is controlled by using the constant voltage circuit 40, themicroprocessor 38 that controls the constant voltage circuit 40, and thetransistor 39.

Specifically, the microprocessor 38 controls the transistor 39 inaccordance with the output voltage Vout2 to set the output potential(potential of the drain electrode 13 d) of the constant voltage circuit40 to the same potential as the potential of the source electrode 13 s.In addition, the microprocessor 38 sets the potential of the siliconsubstrate 11, which becomes the gate electrode 13 g, to the samepotential as the potential of the source electrode 13 s in accordancewith the output voltage Vout1. Then, after passage of a predeterminedtime, the microprocessor 38 control the transistor 39 in accordance withthe output voltage Vout2 to fix the potential difference between thesource electrode 13 s and the drain electrode 13 d to the reversebreakdown voltage (V1) of the Zener diode 40 a. In addition, thecurrent-voltage conversion circuit 41 detects a minute current, whichflows between the source electrode 13 s and the drain electrode 13 d, asa voltage value by removing an in-phase noise, and applies the voltagevalue (input voltage Vin), which is detected, to the microprocessor 38.

The microprocessor 38 controls the potential of the silicon substrate11, which becomes the gate electrode 13 g, through the PID control, forexample, in accordance with a control variable obtained fromcharacteristics illustrated in FIG. 16 so that the voltage value (inputvoltage yin) acquired from the current-voltage conversion circuit 41becomes the constant value. Furthermore, the control variable is storedin the storage unit 38 b (ROM 38 bb). The microprocessor 38 calculatesthe voltage value (output voltage Vout1), which is applied to the gateelectrode 13 g (silicon substrate 11) through the PID control based onthe voltage value (input voltage yin) that is acquired from thecurrent-voltage conversion circuit 41, and the control variable that isstored in the storage unit 38 b.

In addition, the microprocessor 38 applies the output voltage Vout1 tothe gate electrode 13 g. According to this, the microprocessor 38 setsthe current value between the source electrode 13 s and the drainelectrode 13 d to the predetermined value I1. When sensing that thecurrent value between the source electrode 13 s and the drain electrode13 d becomes the predetermined value I1, the microprocessor 38calculates the ion concentration in accordance with the measurementmethod that uses the potential of the silicon substrate 11 that becomesthe gate electrode 13 g, and the characteristics illustrated in FIG. 16.

When the ion concentration is calculated, the microprocessor 38 sets thepotential of the silicon substrate 11 that becomes the gate electrode 13g to the same potential as the potential of the source electrode 13 s inaccordance with the output voltage Vout1. In addition, themicroprocessor 38 controls the transistor 39 in accordance with theoutput voltage Vout2, and sets the potential of the drain electrode 13 dto the same potential as the potential of the source electrode 13 s.

Hereinafter, the description will be given of the measurement method bythe TFT biosensor device with the potential of the source electrode 13 sof the TFT biosensor set as a reference.

FIG. 16 is an explanatory view of a measurement principle in the TFTbiosensor device of the eighth embodiment. In FIG. 16, the horizontalaxis represents the voltage (Vg-s) of the gate electrode 13 g with thepotential of the source electrode 13 s set as a reference. In addition,the vertical axis represents the current that flows between the sourceelectrode 13 s and the drain electrode 13 d. FIG. 16 schematicallyillustrates characteristics of the gate electrode voltage (Vg-s) to thesource-drain current. Here, when sensing the ion concentration, thepotential difference between the source electrode 13 s and the drainelectrode 13 d is fixed to V1 (the reverse breakdown voltage of theZener diode 40 a) as described above.

When the ion concentration in the sensing object material 16 varies, andthe electric double-layer potential difference (Vedl in FIG. 14) at theinterface between the sensing object material 16 and the ion-sensitiveinsulating film 14 varies to +0.1 V, 0 V, and −0.1 V, the microprocessor38 controls the output voltage Vout1 with respect to the siliconsubstrate 11, which becomes the gate electrode 13 g, to 0.5 V, 1 V, and1.5 V so as to allows the current value I1, which is calculated inadvance from the characteristics of the TFT biosensor, to flow. At thistime, the microprocessor 38 calculates an operation amount of the outputvoltage Vout1 with the following Expression (1) so as to stabilize thecontrol. In addition, the microprocessor 38 applies the output voltageVout1, which is calculated, to the gate electrode 13 g (siliconsubstrate 11).Operation amount=Kp×(deviation)+Ki×(accumulated value ofdeviation)+Kd×(difference from immediately previous deviation)  (1)Furthermore, Kp, Ki, and Kd are control variables, and are, for example,0.6, 0.7, and 0.3, respectively. The deviation is a difference betweenthe value (input voltage Vin acquired from the current-voltageconversion circuit 41) that is obtained by reading out the currentbetween the source electrode 13 s and the drain electrode 13 d as avoltage, and a predetermined value that is set in advance.

The output voltage Vout1 from the microprocessor 38 is the same as thevoltage Vg-s of the gate electrode 13 g. Accordingly, in various ionconcentrations, reading-out of the output voltage Vout1 by themicroprocessor 38 in order for the constant current I1 to flow is thesame as reading-out that Vg-s varies in which manner in order for theconstant current I1 to flow. This is none other than sensing of a shiftamount in characteristics of the gate-source voltage to the drain-sourcecurrent in various ion concentrations (as illustrated in FIG. 16). Atable, in which the concentration of hydrogen ions and the gateelectrode voltage Vg-s are associated using a discrete value asillustrated in FIG. 17, is stored in the storage unit 38 b (ROM 38 bb)in advance. Accordingly, the microprocessor 38 specifies theconcentration of hydrogen ions from the output voltage Vout1, which isread out, by using the table.

FIG. 17 is a view illustrating a table in which a correspondence betweenthe concentration of hydrogen ions and the gate electrode voltage Vg-sis stored. The table in FIG. 17 includes a first column in which theconcentration [pH] of hydrogen ions is stored, a second column in whichthe voltage Vg-s [V] of the gate electrode is stored, and a third columnin which the source-drain current I1 [nA] is stored. The tableillustrated in FIG. 17 stores the concentration of hydrogen ions, thegate electrode voltage Vg-s, and the current between the source anddrain electrodes in correspondence with each other. Furthermore, thevoltage Vg-s, which is stored in the table, of the gate electrode 13 gis a voltage so as to set the current, which flows between the sourceelectrode 13 s and the drain electrode 13 d, to the constant current I1,and thus the column of the current between the source and drainelectrodes may not be provided in the table.

As described above, when using the biosensor, which is constituted byusing the thin film transistor described in the first to seventhembodiments, to the TFT biosensor unit S of the eighth embodiment, it ispossible to realize the TFT biosensor device with higher sensitivitycompared with the related art.

Example 12

As a modification example of the eighth embodiment, the description willbe given of a measurement unit in consideration of a variation of theTFT biosensor with the passage of time. When observing the variation inthe drain current of the TFT biosensor of the disclosure with thepassage of time in the test liquid with constant pH under a constantgate voltage and a constant drain voltage, in an ideal state, a constantdrain current is always obtained. However, in many cases, the draincurrent drifts to a direction in which the drain current decreases withthe passage of time.

In an environment in which the drift of the drain current occurs,measurement with high stability is not realized. The drift is caused byslow progress of ion adsorption and ion migration at the interface ofthe ion-sensitive insulating film 14, and it can be said that the driftis a requisite variation for carrying out in-liquid measurement. Asmeans for suppressing the drift, carrying-out of intermittentmeasurement is effective. During a measurement operation, when a voltageis not applied, or an idle period, in which a voltage lower than avoltage during measurement is applied, is provided, it is possible tosuppress the drift during measurement, or it is possible to periodicallyinitialize a sensor state while denying a variation due to the drift.

FIG. 18 is an explanatory view of a measurement method in the TFTbiosensor device of Example 12. In FIG. 18, the horizontal axisrepresents time, and the vertical axis represents a current that flowsbetween the source electrode 13 s and the drain electrode 13 d. FIG. 18represents the variation in the drain current with passage of time inthe TFT biosensor device of Example 12, and a result that is obtained byperforming intermittent measurement by using the TFT biosensor. Forexample, a measurement period in which a predetermined voltage isapplied to the gate electrode 13 g and the drain electrode 13 d,respectively, to measure a drain current, and an idle period in which avoltage that is applied to the gate electrode 13 g and the drainelectrode 13 d is set to 0 V and measurement is not performed, arerepeated for 120 seconds.

With regard to the TFT biosensor of Example 12, first, the semiconductoractive layer 12 that is formed of In—Ga—Zn—O and has the thickness of 50nm, the source electrode 13 s and the drain electrode 13 d which areformed of molybdenum metal and have the thickness of 100 nm, and theion-sensitive insulating film 14 that is formed of tantalum oxide andhas the thickness of 100 nm are formed on the silicon substrate 11covered with the thermal oxide film 10 by sputtering method using metalmask. In addition, after performing annealing in the air at 350° C. forone hour, the protective insulating film 15 formed of silicone resincovers the ion-sensitive insulating film 14 except for a part thereof.

Next, the manufacturing apparatus immerses a thin film transistor in aMcIlvaine buffer solution set to pH 6.0. The McIlvaine buffer solutionis adjusted by 0.05 mM/L of a citric acid aqueous solution and 0.025mM/L of a sodium hydrogen phosphate aqueous solution.

In the TFT biosensor having the above-described configuration, in thecase of being intermittently operated in accordance with the control bythe microprocessor 38, a measurement result illustrated in FIG. 18 isobtained. FIG. 18 illustrates the result obtained by repeating afollowing process cycle in a plurality of times. Specifically, a draincurrent is measured for 120 seconds in a state in which a voltage of 7.5V is applied to the gate electrode 13 g and a voltage of 0.5 V isapplied to the drain electrode 13 d, and then, keeping of a state, inwhich the voltage that is applied to the gate electrode 13 g and thedrain electrode 13 d is set to 0 V, is performed for 120 seconds. In anoperation state in which the voltage of 7.5 V is applied to the gateelectrode 13 g and the voltage of 0.5 V is applied to the drainelectrode, the following state is observed. Specifically, an initialdrain current value of approximately 220 nA is observed, and avariation, in which the drain current value decreases by several nAafter 120 seconds, is observed.

After keeping the gate voltage and the drain voltage to 0 V for 120seconds, when restarting measurement by setting the gate voltage and thedrain voltage to 7.5 V and 0.5 V, respectively, attention is given to avariation in the drain current value. This variation represents that thedrain current value is recovered from a drain current value (valuedecreased from 220 nA by several nA) at the time of termination of theimmediately previous measurement to an initial value of approximately220 nA.

Due to this variation, it is possible to suppress a drift duringmeasurement, or it is possible to periodically initialize a sensor statewhile denying a variation due to the drift.

Furthermore, in the case of assuming that the drift continues even for120 seconds for which the gate voltage and the drain voltage are kept to0 V, it is expected that the drain current value becomes a value thatfurther decreases from the value, which decreased from 220 nA by severalnA, by several nA immediately before restarting measurement. However,even in this case, when restarting measurement, the decrease in thedrain current is canceled, and thus the drain current value is recoveredto the initial value of approximately 220 nA.

In the case of assuming that a time constant of drain currentattenuation is constant, it is possible to realize measurement with highaccuracy in consideration of the above-described drift by reading out adrain current after passage of a constant time from an operation of theTFT biosensor.

In this example, as a voltage condition in the idle period, 0 V isselected for both the gate voltage and the drain voltage. However, thereis no limitation thereto, and only the gate voltage may be set to 0 V,or only the drain voltage may be set to 0 V. The voltage value in theidle period may be a value of 0 V or less, and may be a value of 0 V orgreater as long as the transistor 39 can be controlled in an OFF-state.In this case, the microprocessor 38 controls the ON/OFF state of thetransistor 39 so as to change the drain voltage (voltage between thesource electrode 13 s and the drain electrode 13 d) with a unit timeinterval (for example, for every 120 seconds). In addition, when thedrain voltage is equal to or greater than a predetermined value(threshold value), the microprocessor 38 applies the output voltageVout1, which is calculated based on the voltage value (input voltageVin) acquired from the current-voltage conversion circuit 41, to thegate electrode 13 g. In this case, in a case where the drain voltage isless than the predetermined value, the microprocessor 38 does not applya voltage to the gate electrode 13 g, and thus intermittent measurementbased on a value of the drain voltage becomes possible.

In addition, in this example, the description has been given of theresult obtained by measurement at a cycle of 120 seconds, but anappropriate measurement period may be selected as long as the drift canbe suppressed or canceled. For example, the same effect is obtained witha pulse operation, and an operation can be performed by a sine wave, arectangular wave, and a triangular wave. In addition, for example, whenthe pulse operation is performed at a frequency of 100 Hz or greater, aneffect of suppressing occurrence of the drift is obtained in a pseudomanner, and measurement with high stability can be realized.

Modulation of the operation voltage, and generation of the pulse can beeasily realized by the microprocessor 38 illustrated in FIG. 15.

The circuit configuration and the operation principle, which aredescribed in the eighth embodiment and Examples 11 and 12, areapplicable to the first to seventh embodiments and Examples 1 to 10which correspond thereto. Further, the circuit configuration and theoperation principle are also applicable to an ion sensor (sensor thatdetects a specific ion concentration in a test liquid without usingenzyme reaction) in which the enzyme 19 does not exist in theconfiguration of respective examples.

In addition, the circuit configuration and the operation principle,which are described in the eighth embodiment and Examples 11 and 12, arenot limited to the configuration in which the concentration (pH) ofhydrogen ions varies due to enzyme reaction, and the concentration ofbiomaterial is sensed by detecting the pH variation. The circuitconfiguration and the operation principle are also applicable to aconfiguration of sensing a concentration of an arbitrary ion that isgenerated in the enzyme reaction. For example, in the case of performingsensing of a test liquid in which a cation such as Na ion and K ion isgenerated in the enzyme reaction, a thin film, which is formed by mixinga compound including polypeptide such as valinomycin or crown ether thatbecomes a ligand as a basic skeleton, and a resin material, and byapplying and baking the resultant mixed material, is applicable to theion-sensitive insulating film 14. Even in the case of an anion withoutlimitation to the cation, it is possible to use a ligand that isappropriate to the anion.

According to the disclosure, it is possible to realize the biosensorcapable of detecting with high sensitivity the extremely minute pHvariation caused by enzyme reaction.

In addition, the above-described biosensor is applicable to ahigh-sensitivity biosensor that is used in medical, welfare, andenvironmental fields. When using the oxide semiconductor TFT as aninterfacial potential detection mechanism, it is possible to realizehigh-sensitivity sensing beyond the sensitivity according to the Nernsttheory. In addition, when forming enzyme, which is a biological materialrecognition mechanism, on an insulating substrate that is different froma substrate on which the TFT is formed, application to bio-sensing ispossible without damaging high-sensitivity sensing characteristics. Inaddition, the configuration in which the enzyme is fixed to thedifferent insulating substrate leads to the configuration in whichreplacement of the enzyme substrate is possible, and thus it is possibleto recover a decrease in a function due to inactivation of enzyme. Fromthese characteristics, the biosensor can be used for a disease markerexamination and biomarker examination in a clinical examination.

It is to be noted that the disclosed embodiment is illustrative and notrestrictive in all aspects. The scope of the present technique isdefined by the appended claims rather than by the description precedingthem, and all changes that fall within metes and bounds of the claims,or equivalence of such metes and bounds thereof are therefore intendedto be embraced by the claims.

It is to be noted that, as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise.

What is claimed is:
 1. A biosensor, comprising: a semiconductor activelayer; a first gate insulating film that is provided on a first surfaceof the semiconductor active layer, and insulates the semiconductoractive layer and a first gate electrode from each other; a second gateinsulating film that is provided on a second surface of thesemiconductor active layer; a second gate electrode that is provided onthe second gate insulating film, and extends to a position that istwo-dimensionally spaced away from a region overlapping with thesemiconductor active layer; and an enzyme that is fixed to an extensionend side of the second gate electrode, and reacts with a material in asolution to modulate a voltage that is applied to the second gateelectrode, wherein an electrostatic capacity per unit area of the secondgate insulating film is greater than an electrostatic capacity per unitarea of the first gate insulating film.
 2. A biosensor, comprising: asemiconductor active layer; a first gate insulating film that isprovided on a first surface of the semiconductor active layer, andinsulates the semiconductor active layer and a first gate electrode fromeach other; a second gate insulating film that is provided on a secondsurface of the semiconductor active layer; a second gate electrode thatis provided on the second gate insulating film, and extends to aposition that is two-dimensionally spaced away from a region overlappingwith the semiconductor active layer; an ion-sensitive insulating filmthat is provided on the second gate electrode; and an enzyme that isfixed onto the ion-sensitive insulating film, and reacts with a materialin a solution to allow a potential variation in the ion-sensitiveinsulating film to occur, wherein an electrostatic capacity per unitarea of the second gate insulating film is greater than an electrostaticcapacity per unit area of the first gate insulating film.
 3. Thebiosensor according to claim 2, further comprising: a detection unitthat detects a potential on the ion-sensitive insulating film afteramplifying the potential with a value of a ratio obtained by dividingthe electrostatic capacity per unit area of the second gate insulatingfilm by the electrostatic capacity per unit area of the first gateinsulating film.
 4. A biosensor, comprising: a semiconductor activelayer; a first gate insulating film that is provided on a first surfaceof the semiconductor active layer, and insulates the semiconductoractive layer and a first gate electrode from each other; a second gateinsulating film that is provided on a second surface of thesemiconductor active layer; a second gate electrode that is provided onthe second gate insulating film, and extends to a position that istwo-dimensionally spaced away from a region overlapping with thesemiconductor active layer; an ion-sensitive insulating film that isprovided on the second gate electrode; and a plurality of enzymes whichare fixed onto the ion-sensitive insulating film, and react with amaterial in a solution to allow a potential variation in theion-sensitive insulating film to occur, wherein the plurality of enzymesare disposed on the ion-sensitive insulating film with a regularinterval or in a random manner in order for a surface of theion-sensitive insulating film to come into contact with the solution,and an electrostatic capacity per unit area of the second gateinsulating film is greater than an electrostatic capacity per unit areaof the first gate insulating film.
 5. The biosensor according to claim4, further comprising: a detection unit that detects a potential on theion-sensitive insulating film after amplifying the potential with avalue of a ratio obtained by dividing the electrostatic capacity perunit area of the second gate insulating film by the electrostaticcapacity per unit area of the first gate insulating film.